Speciality Chemicals in Mineral Processing

Speciality Chemicals in Mineral Processing

Speciality Chemicals in Mineral Processing

Edited by D.R. Skuse I M E R YS Minerals, Sandersville, GA, U S A

ROYAL SOCIETY OF CHEMISTRY

The proceedings of the meeting ‘Speciality Chemicals in Mineral Processing’ held at the University of Bath on 25-26 June 2001.

Special Publication No. 282 ISBN 0-85404-831-6

A catalogue record for this book is available from the British Library

0The Royal Society of Chemistry 2002 All rights reserved Apart from any.fair dealing jor the purpose ofresearch or private study, or criticism or review as permitted under the terms of the U K Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any f o r m or by any means, without the prior permission in writing oj The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms c f t h e licences issued by the Copyright Licensing Agency in the UK, or in uccordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org ‘

Typeset in Great Britain by Vision Typesetting, Manchester Printed by MPG Books Ltd, Bodmin, Cornwall, UK

Preface

This review comprises most of the presentations given at the RSC Industrial Affairs Division Speciality Chemicals Sector conference titled ‘Speciality Chemicals in Mineral Processing’ held in Bath, UK in 2001. The papers presented here are not intended to represent a comprehensive review of the use of speciality chemicals in mineral processing but rather the intention is to highlight a significant advance in chemical use in each of a number of different mineral process unit operations. The book is divided into three sections; (i) dispersion and flocculation, (ii) selective processing and (iii) microbiological control. Within each of these areas papers are presented that cover some of the most important recent research and developments from both academia and industry. The first six papers are devoted to dispersion and flocculation. The plenary paper by Hogg reviews ‘The Role of Polymers in Dispersion Stability’ from both a theoretical and practical standpoint and sets the stage for the remaining chapters in the section. In the second chapter, Armes introduces the latest thinking from academia with respect to synthesis of surfactants and dispersants. The ambient temperature atom transfer radical polymerisation techniques described offer considerable opportunities for the future. The third chapter by Heyes reviews recent progress in using molecular simulation to understand the colloidal interactions of concentrated slurries and their role in determining slurry rheological behaviour. The chapters by Duccini and Gole present newly available dispersants and exemplify their behaviour in a number of mineral applications. Finally, the chapter by Phipps presents a colloid chemical analysis of the mechanism by which many polyelectrolyte dispersants work and makes suggestions as to how to improve dispersant performance. There are four chapters on selective processing. The first of these chapters, by Dalton, focuses on using XPS and AFM techniques for the direct study of selective reagent adsorption. Next, Breen presents recent developments in using V

vi

Preface

variable temperature DRIFTS to characterise in detail the interactions of reagents with minerals. This is followed by a chapter by Warne which updates progress on recent work using molecular simulation to construct mineral surfaces and model their interactions with solvents and reagents. The final chapter of the section is by Gorken who describes the development and implementation of hydroxamate-based improved kaolin flotation reagents. The final section of the book is devoted to microbiological control of mineral slurries. The first paper by Duddridge explores the EC’s recent Biological Products Directive and speculates on the challenges that this will create for biocide producers, formulators and mineral slurry processors. The final two papers by Gutherie and Martin both introduce new microbiological control regimes. It is a great pleasure to acknowledge all the contributors to this volume, without whom the conference and this book would not have been possible. In addition, it is a pleasure to acknowledge the support of the exhibitors who contributed to the success of the conference from which this book is derived, namely; Rohm and Haas, Ondeo-Nalco, Quantachrome, and Scientific and Medical products Ltd. I am grateful to Elaine Wellingham of the RSC for administrative help and to Dr Susan Partridge for considerable help with the manuscript. Finally and particularly, acknowledgement must go to my employer IMERYS Minerals (formerly English China Clays) for supporting this endeavour and for direct financial support for four of the projects reported in this volume.

Contents

Dispersionand Flocculation The Role of Polymers in Dispersion Stability R. Hogg and C. Rattanakawin Synthesis of New Polymeric Surfactants and Dispersants Via Atom Transfer Radical Polymerisation at Ambient Temperature S.P. Arrnes, K.L. Robinson, S.Y. Liu, X.S. Wang, F.L.G. Malet and S.A. Furlong

3

21

Role of Colloidal Interactions in Determining Rheology D.M. Heyes and J.F.M. Lodge

31

New Polymeric Dispersants for Very Fine Calcium Carbonate Slurries Y. Duccini and A . DuFour

38

AMP-95TMfor Mineral Dispersion T. Gole

47

Pigment Dispersion Technology for the Paper Industry D.J. Mogridge, J.S. Phipps, K.R. Rogan and D.R. Skuse

55

Selective Processing An AFM and XPS Investigation of the Selective Flocculation of Kaolinite from a Mineral Mixture J.S. Dalton, G.C. Allen, K.R. Hallam, N.J. Elton, J.J. Hooper and D.R. Skuse vii

71

...

Contents

Vlll

The Nature of Adsorption Sites on Unrefined and Ball Milled Kaolin. A Diffuse Reflectance lnfrared Fourier Transform Spectroscopic Study C. Breen, J. Illis, J . Yarwood and D.R. Skuse Computer Simulation of Water Molecules at Mineral Surfaces M . Warne, N.L. Allan and T. Cosgrove Modified Hydroxamate Collectors for Kaolin Flotation C. Basilio, R.A. Lowe, A. Gorken, L. Magliocco and R. Hagy

81

90

102

Microbiological Control The Biocidal Products Directive and the Mineral Processing Industry: Possible Issues and Implications J. Duddridge Microbiological Problems in Mineral Slurries - Control Options in 2001 W.G. Guthrie and D.W. Ashworth

111

123

Modern Microbiological Problems and Solutions for the Pigment Slurry Industry J.L. Martin

133

Subject Index

141

Dispersion and Flocculation

The Role of Polymers in Dispersion Stability R. Hogg and C . Rattanakawin DEPARTMENT OF ENERGY AND GEO-ENVIRONMENTAL ENGINEERING, THE PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, P A 16802, USA

1 Introduction Soluble polymers are widely used to control the state of dispersion of fineparticle suspensions. Depending on the polymer, and how it is applied, they can serve to enhance stability (dispersants) or to promote aggregation of the particles (flocculants). The topics covered in this chapter are intended as an overview of the use of polymers for stability control in mineral-particle suspensions with particular emphasis on flocculation processes. A brief discussion of stabilisation by polymers is included for completeness.

1.1 Polymeric Dispersants Both natural and synthetic polymers with molecular weight up to about 20 000 are commonly used for stabilisation of fine-particle dispersions. It is generally accepted that these reagents function by presenting a steric barrier to direct contact between particles (steric stabilisation).' In effect, adsorption of the soluble polymer provides a lyophilic film on the particle surfaces that prefers contact with the solvent to contact with a similar film on another particle. The basic requirement for steric stabilisation is that the adsorbed layer be thick enough to prevent particles from approaching each other closely enough for attractive forces (e.g. van der Waals) to cause adhesion. More or less complete coverage of the surfaces is generally necessary. Charged polymers (polyelectrolytes) can further enhance the effect by adding an electrostatic repulsion (electrosteric stabilisation). Addition of the electrostatic component may serve to relax the coverage requirements for purely steric stabilisation.

3

4

The Role of Polymers in Dispersion Stability

1.2 Polymeric Flocculants Polymers used to promote flocculation normally have higher molecular weight than the dispersants - usually from about 50 thousand up to about 20 million. Most of those in use today are synthetic, linear-chain molecules, and include non-ionic, anionic and cationic types. Non-ionic flocculants are commonly polyacrylamide, while the anionic reagents are often acrylamide/acrylate copolymers. Typical non-ionic and anionic flocculants have high molecular weights - in the 10 to 20 million range. Cationic flocculants (sometimes referred to as coagulants) are normally of lower molecular weight ( < 1 million) and commonly owe their charge to the presence of a quaternary ammonium group. The polydiallyldimethylammonium chloride (DMDAAC) types are widely used commercially. Charge densities are usually substantially higher for the cationic reagents than for the anionic types. Flocculation by polymers, like dispersion, normally involves adsorption at particle surfaces. The high molecular weight flocculants are generally agreed to function through a ‘bridging’ mechanism whereby a single large molecule can attach to two or more particles simultaneously, providing a physical link bridge - between them.2-4 Smaller, highly charged polyions can adsorb on oppositely charged particle surfaces forming patches of opposite charge. Thus, a cationic polymer can form positively charged patches on the surface of negatively charged particles. Interaction of a patch on one particle with a region of bare surface on another then leads to a g g r e g a t i ~ nA . ~ third mechanism, known as depletion flocculation, does not involve adsorption. Rather, the exclusion of large polymer molecules from small spaces between particles in concentrated suspensions promotes aggregation through osmotic pressure effect^.^

1.3 Polymer Solutions The properties of polymer solutions play an important role in the dispersion or flocculation of fine-particle suspensions. Individual molecules in solution generally take on a more-or-less randomly coiled configuration. Repulsion between similarly charged ionic groups in a polyelectrolyte leads to expansion of the coil, increasing the effective size of the molecule in solution. Modification of the charge, e.g. by protonation of acid groups in an anionic polymer, gives rise to changes in molecular conformation. Shielding of the charges in the presence of simple electrolytes generally leads to increased coiling, i.e. smaller effective molecular size.

2 Polymer Adsorption The adsorption of polymers at solid surfaces is substantially more complicated than that of small molecule^.^>^ Typically, a small molecule adsorbs by attachment of a functional group on the molecule to a site on the surface. For the case of adsorption from aqueous solution, the process involves displacement of water from sites at the surface. The extent of adsorption is determined by the preference

R . Hogg and C. Rattaizakawin

5

of the surface site and the adsorbing species for contact with water rather than with each other. While the same is generally true for polymers, the presence of a large number of potential attaching groups on each molecule biases the system towards adsorption. A typical polyacrylamide flocculant with a molecular weight of 10 million has about 140000 individual segments, each of which is capable of attaching to the surface. Only one such attachment is necessary for adsorption of the whole molecule. The consequences of this ‘multiple adsorption’ effect are: Polymer adsorption tends to be indiscriminate. The large molecules become attached to essentially any available surface unless strongly repelled from it. Adsorption is effectively irreversible. Desorption of the molecule requires simultaneous detachment of all adsorbed segments. Since the adsorption of individual segments is quite reversible, the conformation of the adsorbed molecules can vary continuously and the molecules are free to migrate over the solid surface. The strength of the segment-surface site interaction has more effect on the conformation of the adsorbed molecules than on the number of molecules adsorbed. The effective ‘parking area’ of the adsorbed molecule may vary with the extent of adsorption. As more molecules are adsorbed, the area occupied by each may decrease due to a crowding effect. When the adsorbing polymer acts as a flocculant, the adsorption process is further complicated by concurrent aggregation of the particles. Adsorption of the polymer provides the linkages needed for bridging flocculation while the formation and growth of aggregates reduces the surface area available for adsorption. An interesting consequence is that effective flocculation actually reduces the amount of polymer An example of this inverse relationship is given in Figure 1. Adsorption densities at pH 4.5 and 11, where the dispersion was initially stable and flocculation by the polymer was poor, are more than ten times greater than those on the flocculated suspension at pH 9. Because polymer adsorption is effectively irreversible, and because adsorption and floc growth occur simultaneously, flocculation is a non-equilibrium process. As a result, performance is largely determined by the kinetics of adsorption and aggregation. Both of these can be regarded as collision processes involving solid particles and polymer molecules. In each case, collisions can arise due to either Brownian motion or agitation of the suspension. The collision frequency v between particles and polymer molecules can be estimated from:”

v

= Knpn

(1)

where n and np are the respective number concentrations for solid particles and polymer molecules and K is a rate constant. For Brownian motion: K=-

2kT 3P

x

a)

l+>+-

The Role qf Polymers in Dispersion Stability

6 10

L

// I

0

0

v

I

i

pH 4.5 pH9 pH11

v

0.01 0.01

0.1

1

10

Residual Polymer Concentration, mg/L

Figure 1 Efect of p H on the adsorption of a nun-ionic polyacrylumide o n j n e alumina particles (after Ray)'O

where k is Boltzmann's constant, T is absolute temperature, p is the fluid viscosity and x and xp are the sizes of the particles and molecules in solution. In the case of collisions due to agitation: K

G 6

= -(x

+ xp)3

(3)

where G is the mean shear rate due to agitation. The relative importance of the two mechanisms can be seen in Figure 2. The curves indicate, for different shear rates, molecular weight/particle size combinations for which the rates are equal. Combinations to the right of the lines represent domination by shear. It is clear that the rates are dominated by the shear mechanism for most conditions of practical interest.

3 Polymers and Dispersion Stability Dispersions of fine mineral particles can be stabilised by direct electrical charging of the particles or by steric/electrosteric protection from adsorbed polymers. Stabilisation by direct charging is well described by the classical DLVO theory.' '

3.1 Steric Stabilisation Dispersion stabilisation generally requires fairly complete coverage of particle

R. Hogg and C. Rattanakawin

7

107

105

0.1

1

10

100

Particle Size, pm

Figure 2 Relative adsorption rates due to collisions resulting from Brownian motion and mechanical agitation. The lines represent conditions where the rutes are equal. Agitation dominates to the right of the lines

surfaces by adsorbed polymer. Polymer molecular weight should be sufficiently large to prevent close approach of the particles, but not so large as to promote bridging flocculation. Purely steric stabilisation usually involves high polymer dosage - of the order of 1 mg per square metre of solid surface.

3.2 Electrosteric Stabilisation In many practical cases, stabilisation by polymers involves a combination of steric and charge interactions. Unlike simple electrolytes, multiple adsorption effects permit polyelectrolytes to continue to adsorb well beyond the point where the adsorbed layer charge exceeds that of the particle surface. In this way, the effective charge on particles can be increased substantially at relatively low surface coverage by the polymer.

4 Flocculation by Polymers In general, flocculation processes involve two basic steps: Destabilisation of the dispersion to permit particle-particle collisions and aggregate formation. Growth of small aggregates to form large flocs. Polymers can play a role in both of these.

8

The Role of Polymers in Dispersion Stability

4.1 Destabilisation The stability of suspensions of fine mineral-particles usually results from the electrical repulsion forces that prevent association of particles into aggregates. These can be minimised by reducing surface potentials - often by appropriate pH adjustment - or by shielding of the charge using simple electrolytes. In principle, polyelectrolytes can also serve to shield the charge on particle surfaces, but the charge-patch and bridging mechanisms described above are probably more important in practice. An example of destabilisation by a polyelectrolyte, probably through the charge-patch mechanism, is given in Figure 3. The figure shows the evolution of the floc size distribution with the addition of a cationic polymer (molecular weight about 100000) to a suspension of fine alumina particles in water at pH 11. Initially the dispersion was stable due to the negative charge on the particles at that pH. Upon addition of the polymer, the distribution first becomes bimodal, consisting of small flocs and some residual primary particles. As more polymer is

4

d 10

I

1

.

*--

8 -

.

4 -

2 -

-

0

*

0-

0.54 mg/L

... . ... . .-. *

6 -

,

:

.

,

,

, . I

L,

. , . .:

Figure 3 Destabilisation of a$ne alumina suspension (3% solids by weight) at p H 11 by a cntionic polyelectrolyte (molecular weight about 100 000)

9

R. Hogg and C. Rattanakuwin

added, the primary-particle mode disappears and the distribution becomes unimodal again at a polymer concentration of about 0.5 mg L-I (0.0167 mg per gram of solids). However, further polymer addition has very little effect, other than increasing the median floc size from about 6-7 pm at 0.5 mg L-' to about 9 pm at 5 mg L- l. Apparently, the cationic polymer is effective for destabilising the suspension but not for promoting floc g r o ~ t h . ~ ~ . ' ~ Very similar effects can be seen for higher molecular weight (about 15 million) non-ionic and anionic polymers. Some results for the addition of a non-ionic polyacrylamide to the same alumina under the same conditions are given in Figure 4. The unimodal-bimodal-unimodal progression can be seen again. The principal differences from the cationic polymer are that more polymer is needed to eliminate the primary-particle mode (about 3 mg L-') but continued polymer addition leads to significant growth of the flocs - up to about 80 pm at 5 mg L-I of the polymer. For the anionic polymer, even more polymer is required to eliminate the primary particles (10mgL-l) and floc growth continues up to more than 200 pm. It seems that the higher molecular weight polymers are less

1.07 m g L ] 4

L

.

2 ~&

0 0.1

1

#**.

10

,

. .,

, , , I

100

*:\.

. .: ,

1000

Size (pm)

Figure 4 Destabilisution andjocculation of afine ulumina suspension (304 solids by weight) at pH 11 by a non-ionic polyelectrolyte (molecular weight about 15 million)

10

The Role of Polymers in Dispersion Stability

effective for destabilisation of the primary particles but more effective for promoting floc growth. The anionic polymer is able to induce flocculation despite the similar charge on the polymer and the particles. Apparently, the high molecular weight permits adsorption and bridging to occur. It is interesting to compare destabilisation by polymers with equivalent results for simple electrolytes. The effects of adding calcium chloride to the alumina suspension at pH 11 are shown in Figure 5. At a salt concentration of about 1 0 - 4 m ~ l L-' e ~ (11mg L-'), the size distribution changes abruptly from that of the primary particles to a unimodal floc size distribution centred at about 6 ym. Destabilisation of the alumina at pH 5 where the particles are positively charged shows quite similar trends, the principal difference being that the cationic polymer is completely ineffective.I5Presumably, the positive charges on both particles and polymer are sufficient to prevent adsorption of the relatively small, highly charged polyions. The behaviour of the non-ionic and anionic polymers is essentially the same as at pH 11.15Destabilisation by the addition of

12

1 t

16

I

1

No CaCI, addition

a

a.

.

1

1

I

0

a

. . a

L

4 -

a

a*.

0.1

1

,

a

a a

. .,,

a

,:aa-

10

Size (pn)

Figure 5 Destabilisation of ajine alumina suspension (3% solids by weight) at pH 11 by the addition of a divalent cation

R. Hogg and C . Rattanakawin

11

divalent sulfate ions at pH 5 is virtually identical to that observed for calcium at pH 1l.15Adjustment of the pH to around the point of zero charge (pH 9) leads to the same floc size distributions as the simple salt additions.I5

4.2 Floe Growth Collisions and adhesion between particles/flocs lead to floc growth. The collision mechanisms described previously for polymer adsorption apply equally to floc formation and growth. Collisions due to shear can be expected to dominate for most practical conditions. In order for growth to continue, it is necessary for the collisions to lead to adhesion and for the resulting flocs to be resistant to degradation and breakage in the agitated system. Destabilisation of the dispersion should ensure that particles adhere on contact, but the forces involved will not necessarily be sufficient to withstand disruption of a large floc subjected to shear. In processes for deaggregation or breakage of particles under applied stress, it is generally observed that breakage rates increase with increasing particle or aggregate size. It follows that simultaneous growth and breakage in flocculation processes can be expected to lead to the approach to a limiting floc size, at which growth and breakage rates are equal. Stronger flocs should grow to larger limiting sizes. It is postulated that polymers can perform a dual function in flocculation. They can aid in destabilisation as noted above and also serve as binders in the growing flocs. The results shown in Figures 3-5 are clearly consistent with this concept. Destabilisation by pH control or simple salt addition is sufficient to allow individual particles to adhere on contact, but the resulting flocs are weak and can only grow up to a relatively small limiting size (about 6ym for the conditions applying in Figure 5). The use of a polymer produces stronger flocs as reflected in the larger limiting sizes shown in Figures 3 and 4: over 100pm for the high molecular weight non-ionic and anionic polymers. The low molecular weight cationic polymer appears to provide only a minor increase in floc strength. It is apparent that pH control or the addition of simple salts or low molecular weight polyelectrolytes is typically sufficient for destabilising fine-particle dispersions, while high molecular weight polymers are necessary to permit floc growth to large sizes. It follows that the use of reagent combinations should offer considerable potential for efficient flocculation. Some examples of this approach are illustrated in Figures 6 and 7. The effects of pH control and salt addition can be seen in Figure 6. Addition of a non-ionic polymer directly to the stable dispersion requires 9-10 mg L-l to produce flocs with a median size of about 50 pm. By first adding lo-* moles L-' of sulfate ions or adjusting the pH to about 8, the same floc size can be obtained with a polymer addition of less than 2mgL-l. The use of polymer combinations is demonstrated in Figure 7. The addition of about 1mgL-' of a low molecular weight cationic polymer to negatively charged alumina particles at pH 11 destabilises the dispersion but produces only very small flocs - around 7pm (Figure 7a). Further treatment, with about

12

The Role qf Polymers in Dispersion Stability

l2

b) PH 5, lo-*M Na,SO,

m 9.63 mg/L

rn 1.61 mg/L

l2

I

12

-

- ....... . ..... "

1.61 mg/L

3 ... .. -

8 -

-

4-

0

0

0.1

1

10

100 1000 0.1

Size (pn)

-

1

10

.

100 1000

Size (pm)

Figure 6 Eflect of destabilisution by salt addition or pH control on flocculation with a non-ionic polymer

2 mg L-l of a high molecular weight anionic polymer, causes substantial floc growth - to over 100pm (Figure 7b). In the use of these reagent combinations, where destabilisation and floc growth are addressed separately, the order of treatment is extremely important. Destabilisation must be achieved first, before the growth-enhancing polymer is applied. An example of the effect of reversing the sequence can also be seen in Figure 7. Addition of about 2 m g L - 1 of the anionic polymer to the stable dispersion produces a bimodal floc size distribution consisting of a mixture of flocs at around 30 pm and primary particles at around 0.5 pm (Figure 7c). The addition of 1 mg L- of the cationic polymer causes aggregation of the residual primary particles, shifting the fine-particle mode to about 10pm but has only minor effects on the coarser mode (Figure 7d). If anything, addition of the cationic polymer seems to reduce the height of the coarse mode, implying a transfer of particles from the coarse to the fine modes.

13

R. Hogg and C. Rattanakuwin

0.1

1

10

100

1000

0.1

1

10

100

1000

0.1

1

10

100

1000

10

100

1000

10

6 8

a -

2

6

L

0 0.1

1

Size (pm)

Size (pm)

Figure 7 Destabilisation/focculation of a fine alumina suspension at p H 11 by polymer combinations. (a) Cationic only (1.07 mg L-I); ( b ) cationic (1.07 mg L - ' ) followed by anionic (2.1 4 mg L-I); (c) anionic only (2.1 4 my L-'): (d) anionic (2.14 mg L - * )followed by cationic ( I .07 mg L - ' )

5 The Flocculation Process 5.1 Mixing Chemical treatments to induce flocculation in fine-particle dispersions are generally sensitive to the process of adding reagent^.^.^^.'^ Mixing of reagents with the suspension is usually accomplished by some kind of mechanical agitation. It is important to recognise, however, that agitation also promotes both growth and breakage of flocs. As noted above, the balance between growth and breakage rates typically leads to the approach to a limiting floc size. For systems involving only simple reagents such as inorganic salts, chemical equilibrium is probably reached also. On the other hand, the effective irreversibility of polymer adsorption makes the establishment of equilibrium unlikely for cases of polymerinduced flocculation. In fact, it can be demonstrated that the approach to equilibrium is usually detrimental to the process and that efficient flocculation involves exploiting the non-equilibrium nature of the process. Because polymer adsorption tends to be irreversible, inadequate mixing, with non-uniform distribution of polymer in the particle suspension, cannot be rectified by subsequent rearrangement. Locally high concentrations can lead to rapid Aoc growth, but with corresponding regions of starvation elsewhere in the

14

The Role of Polymers in Dispersion Stability

system. The problem is especially acute at high solids concentrations because of the very high rates of adsorption. It follows that the time required for polymer adsorption should be greater than the ‘mixing time’. For stirred tanks, the mixing time t, can be estimated from:I7 (4)

t, z 36/N

where N is the rotational speed of the impeller. Thus, for the mixing system used in the experiments described above ( N = lOOOrpm), mixing times of around 2 seconds can be expected. The corresponding adsorption time can be estimated using Equations 1 and 3. The number concentration of particles is given by: n = - (P kvx3

where (P is the volume fraction of solids in suspension and kV is the volume shape factor. Neglecting the molecular size xp in Equation 3, the adsorption rate (assumed to be equal to the collision frequency) can be estimated from:

where C, and C, are respectively the concentrations of adsorbed and free polymer in the system. If the overall polymer dosage is C,,, added at time zero:

c, = c,, - C ,

(7)

and the solution to Equation 6 is:

c, = c,,

[

1 - exp

(

-

t)]

For the results shown in Figures 3-6, G z 1000sec-’ and (P = 0.01. Assuming roughly spherical particles, k , z 7116. The time needed for 90% of the polymer to be adsorbed would be about 3 seconds, which is quite similar to the mixing time. In other words, a significant fraction of the added polymer could be adsorbed locally before the mixing action could provide a uniform distribution. In order to minimise the detrimental effects of localised adsorption, it is generally preferable to add the polymer continuously over an extended period, substantially greater than the mixing time. Since the same mechanisms are involved, aggregation times are generally similar to adsorption times. As a consequence, floc growth rates typically parallel adsorption rates or, for continuous polymer addition, the rate of polymer addition. Furthermore, floc growth continues only so long as fresh polymer is

R. Hogg and C. Rattanakawin

15

available in solution. Prolonged agitation, in the absence of fresh polymer leads to degradation of the f l o ~ s . ~ ~ 7 ' ~

5.2 Solids Concentration The concentration of suspended solids is an important variable in flocculation and can vary from parts per million in potable water treatment to more than 20% by volume in filtration applications. Differences in solids concentration affect both the amount of polymer needed and the rates of adsorption and floc growth. Dosage requirements can reasonably be expected to vary in direct proportion to solids concentration but the constraints on the mode of polymer addition appear to be less simple. At high solids concentrations, the potential floc growth rate will invariably be higher than any reasonable rate of continuous polymer addition. Consequently, growth is controlled by the rate of addition. However, the limiting floc size is also affected by solids concentration. In addition to the breakage limitation described previously, floc size in very concentrated suspensions may be limited by a crowding effect. It is well known that floc structures become increasingly open as size increases. Thus, as flocs grow, they eventually approach a condition of close packing, thereby preventing further growth. Somewhat different constraints apply at low solids concentrations. Adsorption and floc growth rates are much lower and mixing is a less critical factor. However, while growth rates are smaller, breakage rates are determined mostly by floc size and agitation intensity and should be relatively unaffected by solids concentration. The balance between growth and breakage can then be expected to occur at a smaller size. It follows that, at low solids concentration, it may be appropriate to employ lower agitation rates and allow an extended mixing period.

5.3 Polymer Solution Concentration The concentration of the polymer solution added to the particle suspension has also been shown to affect process perf0rman~e.l~ The use of more dilute solutions appears to enhance floc growth. In concentrated solutions, intermolecular repulsion enhances coiling of the molecules, reducing their effective size. Because of the very high adsorption rates, there is insufficient time for relaxation of the molecules, and the surface covered by a molecule and its extension into solution are both reduced.

6 Applications The observations described above are based on small-scale laboratory testing. Application to industrial operations such as mineral processing introduces further questions. While chemical requirements can probably be evaluated directly from the laboratory tests, the physical aspects, particularly mixing and

16

The Role of Polymers in Dispersion Stability

reagent addition, must be scaled-up appropriately. Furthermore, while laboratory testing is normally carried out in the batch mode, industrial processes are usually continuous.

6.1 Scale-up Laboratory tests indicate that agitation intensity and the rate of polymer addition are important factors in determining process performance. Information on scale-up rules for these variables is, unfortunately, rather limited. Maffei1'>l carried out an empirical study of the flocculation of kaolin with polyacrylamide in a series of geometrically similar stirred tanks ranging in volume from 0.3 to 52 litres. His results suggest the following rough guidelines for the scale-up of batch flocculation systems based on laboratory tests: Maintain the same overall polymer dosage and relative rate of addition (rate per unit volume). For continuous polymer addition this specifies the same mixing/reagent addition time. Scale the agitator speed to maintain the same impeller tip speed. Thus, for an impeller of diameter d: N

K

l/d

(9)

It should be emphasised that these are approximate guidelines only, based on quite limited experimental data.

6.2 Continuous Flocculation Most industrial flocculation is carried out as a continuous process, either in a stirred tank or, perhaps more commonly, by direct injection of a polymer solution into a flowing stream. Again, only rather limited information is available on the behaviour and performance of such systems. A study of continuous flocculation in a stirred tank2072' has revealed some similarities and some differences relative to the equivalent batch system. An important inherent difference is that, whereas polymer addition in a batch system leads to increasing dosage over the period of addition, the polymer content of a continuous system at steady state is constant with time. It has been postulated that this difference permits a somewhat closer approach to equilibrium in the continuous system. The result is that, while similar trends are observed, the effects of mixing/reagentaddition time are much less dramatic for the continuous case. A comparison of the effects of mixing time on batch and continuous flocculation is given in Figure 8. In general, continuous flocculation appears to produce smaller, slower-settling flocs than the batch systems. Flocculation during flow through pipes has also received some a t t e n t i ~ n . ~ ~ , ~ ~ To some extent, laboratory-scale test results parallel those obtained in stirred tanks. In principle, the addition of polymer at a single location in a pipe is roughly analogous to instantaneous addition followed by a period of continued

17

R. Hogg and C. Rattanakawin I

.

I

.

,

,

.

*

. . ...,

.

. . . . . ..,

.

. .

0.2 -

0.0

~

1

10

100

Mixing Time, sec

Figure 8 A comparison oj'the efleects of mixing time on batch and continuousflocculation of kaolin at 3% solids by weight by 4mgL-I of a non-ionic polymer (after Su haryono)22

mixing in a stirred tank, operated in the batch mode. Similarly, addition at several points along the length of the pipe is equivalent to continuous addition to a tank. Experimental results are consistent with these similarities but, as for continuous operation in tanks, the flocs produced are generally smaller and exhibit lower settling rates than those formed in batch systems.25 Direct comparison between batch and continuous, in-line flocculation at the laboratory-scale is difficult. Since mixing action in pipes is provided by flow, mixing intensity and mixing time cannot be varied independently in a pipe of fixed length. Achieving high mixing intensities similar to those found to be appropriate in stirred tanks requires high flow rates through pipes of small diameter. Extremely long pipes are then needed in order to obtain similar mixing times. The same problem of matching mixing times and agitation intensities presents serious difficulties in scaling up to industrial conditions.

7 Conclusions Polymers can be used either to stabilise fine-particle dispersions or to promote flocculation. Adsorption of the polymer at particle-solution interfaces is usually involved in both cases. The principal differences between the two are the molecular weight and relative amounts of polymer needed: high surface coverage by a low molecular weight polymer leads to stabilisation while flocculation requires much lower dosage of high molecular weight polymers. In the case of dispersion stabilisation, adsorption occurs on individual particles, which remain as discrete entities. Consequently, the process can be conducted under near-equilibrium conditions and performance depends largely on chemical factors. Flocculation,

18

The Role of Polymers in Dispersion Stability

on the other hand, involves polymer attachment to more than one particle, accompanied by aggregation of the particles. The process occurs under nonequilibrium conditions and performance is strongly affected by physical factors such as mechanical agitation. In general, flocculation involves two stages - destabilisation of the suspension and the growth of flocs. Destabilisation can be achieved by: pHcontrol appropriate salt addition the use of low molecular weight polyelectrolytes High molecular weight polymers are not recommended for destabilisation but can be highly effective in promoting floc growth in already destabilised systems. The order of treatment - destabilisation followed by flocculant addition - is very import ant. The performance of high molecular weight polymeric flocculants is heavily dependent on the physical conditions of their application. Recommended treatment conditions are: addition from a dilute polymer solution rapid addition with vigorous agitation (short mixing time) at low solids concentrations ( < 1% by volume), a follow-up period of relatively gentle agitation is useful to allow for floc growth continued agitation of a concentrated suspension after flocculant addition serves mainly to degrade existing flocs and should be avoided Scale-up considerations for batch flocculation in stirred tanks indicate that similar results are obtained with: the same rate of polymer addition per unit volume the same mixing/polymer-addition time the same impeller tip speed Continuous flocculation in stirred tanks shows similar trends with regard to agitation/mixing conditions as for batch systems. The overall performance of continuous flocculation processes tends to be inferior to that which can be obtained in batch systems. This is attributed to a closer approach to equilibrium in the continuous case. In-line flocculation, by injection of polymer into turbulent flow in a pipe, is widely practised in industry and offers certain advantages. Multiple addition at several points along the length of a pipe is preferred to single-point addition.

Nomenclature C, C, C,, d G

Concentration of adsorbed polymer Concentration of polymer in solution Initial polymer concentration Impeller diameter Mean shear rate

ML-3 ML-3 M L-3 L T-

R . Hogg and C. Rattanakuwin

Boltzmann’s constant Volume shape factor Collision rate constant Number concentration of particles Number concentration of polymer molecules Impeller rotational speed Time Mixing time Absolute temperature Particle size Effective size of polymer molecule Solids concentration (volume fraction) Liquid viscosity Collision frequency

19

ML2 T - 2

O K -

1

-

L3T-l L-3 L-3 T-’ T T OK L L -

M LT-l L-3~-1

References 1. D.A. Napper, Polymeric Stubilisation of Colloidal Dispersions, Academic Press, New York, 1983. 2. A.S. Michaels, Ind. Eng. Chem., 1954,46, 1485. 3. V.K. La Mer and T.W. Healy, Rev. Pure Appl. Chem., 1963,13, 112. 4. J.A. Kitchener, Br. Polymer J., 1972,4,217. 5. J. Gregory, J . Coll. Interface Sci., 1973,42,448. 6. G.J. Fleer, J.M.H.M. Scheutjens and B. Vincent, ‘The Stability of Dispersions of Hard Spherical Particles in the Presence of Nonadsorbing Polymer’ in Polymer Adsorption and Dispersion Stability, E.D. Goddard and B. Vincent (eds.), American Chemical Society, Washington DC, 1984, ACS Symposium Series 240, Chapter 16, pp. 245-263. 7. P.J. Flory and T.G. Fox Jr., J . Am. Chem. Soc., 1951,73, 1904. 8. J.M.H.M. Scheutjens and G.J. Fleer, J . Phys. Chem., 1979,83,1619. 9. M.A. Cohen-Stuart, T. Cosgrove and B. Vincent, Adv. Coll. Interface Sci., 1986, 24, 143. 10. D.T. Ray, PhD Thesis, The Pennsylvania State University, 1988. 11. D.T. Ray and R. Hogg, ‘Bonding of Ceramics Using Polymers at Low Concentration Levels’ in Innovations in Materials Processing Using Aqueous, Colloid & Surfuce Chemistry, F.M. Doyle, S. Raghavan, P. Somasundaran and G.W. Warren (eds.),The Minerals, Metals & Materials Society, Warrendale PA, 1989, pp. 165-180. 12. R. Hogg, Colloids and Surfaces A , 1999,146,253. 13. J.Th.G. Overbeek, in Colloid Science, H.R. Kruyt (ed.), Elsevier, Amsterdam, 1952, Vol. 1, Chapters V1 and VII, pp. 245-301. 14. C. Rattanakawin, MS Thesis, The Pennsylvania State University, 1998. 15. C. Rattanakawin and R. Hogg, Colloids and Sur$aces A , 2001,177,87. 16. R.O. Keys and R. Hogg, ‘Mixing Problems in Polymer Flocculation’in Water - 1978, G.F. Bennett (ed.), AIChE Symposium Series, American Institute of Chemical Engineers, New York, 1979, Vol. 75, pp. 63-72. 17. R. Hogg, P. Bunnaul and H. Suharyono, Miner. Metall. Process, 1993,10, 81. 18. R. Hogg, R.C. Klimpel and D.T. Ray, Miner. Metall. Process, 1987,4, 108. 19. L.A. Cutter, AIChE J . , 1966,12, 35. 20. A.C. Maffei, MS Thesis, The Pennsylvania State University, 1989.

20

The Role of Polymers in Dispersion Stability

21. R. Hogg, A.C. Maffei and D.T. Ray, ‘Modeling of Flocculation Processes for Dewatering System Control’ in Control ’90 Mineral and Metallurgical Processing, R.K. Rajamani and J.A. Herbst (eds.), Society for Mining, Metallurgy and Exploration, Littleton CO, 1990, Chapter 4, pp. 29-34. 22. H. Suraryono, PhD Thesis, The Pennsylvania State University, 1996. 23. H. Suharyono and R. Hogg, S M E Preprint No. 94-231, Society for Mining, Metallurgy and Exploration, Littleton CO, 1994. 24. J. Gregory and L. Xing, ‘Influence of Rapid Mix Conditions on Flocculation by Polymers’ in Dispersion and Aggregation: Fundamentals and Applications, B.M. Moudgil and P. Somasundaran (eds.),Engineering Foundation, New York, 1994, pp. 427-441. 25. H. Suharyono and R. Hogg, Miner. Metall. Process, 1996,13,93. -

Synthesis of New Polymeric Surfactants and Dispersants Via Atom Transfer Radical Polymerisation at Ambient Temperature S.P. Armes, K.L. Robinson, S.Y. Liu, X.S. Wang, F.L.G. Malet and S.A. Furlong SCHOOL OF CHEMISTRY, PHYSICS AND ENVIRONMENTAL SCIENCE, UNIVERSITY OF SUSSEX, FALMER, BRIGHTON, EAST SUSSEX, BN19QJ, UK

1 Introduction Atom Transfer Radical Polymerisation (ATRP) was discovered independently by Wang and Matyjaszewski,' and Sawamoto's group2 in 1995. Since then, this field has become a 'hot topic' in synthetic polymer chemistry, with over 1000 papers published worldwide and more than 100 patent applications filed to date. ATRP is based on Kharasch chemistry; overall it involves the insertion of vinyl monomers between the R-X bond of an alkyl halide-based initiator. At any given time in the reaction, most of the polymer chains are capped with halogen atoms (Cl or Br), and are therefore dormant and do not propagate; see Figure 1. The role of the copper-based ATRP catalyst is to remove these terminal halogen atoms and generate polymer radicals. A short burst of propagation occurs, followed by rapid, reversible halogen capping of the growing polymer chain. Thus the instantaneous polymer radical concentration is significantly reduced relative to that normally found in conventional radical polymerisations. This means that the probability of any two polymer radicals terminating is suppressed relative to the probability of propagation (chain extension). Hence remarkably narrow molecular weight distributions can be achieved, in some cases as low as 1.05 for p ~ l y s t y r e n e . ~ ATRP has been used to polymerise styrenic, acrylic and methacrylic monomers. However, conventional ATRP of styrenic and methacrylic monomers is rather sluggish: typically many hours are required at high temperatures ( >90 "C) for incomplete conversions ( < 90%) even under bulk polymerisation conditions. Acrylates polymerise much faster than methacrylates or styrenics and the rapid, efficient polymerisation of n-alkyl acrylates at room temperature with various 21

22

Synthesis ($New Polymeric Surfactants and Dispersants

-R ~ M P M

R-x

P-x

+

X-Cu(I)k

ATRP

\

-

P* +

w f

\

X-Cu(II)L2 I X

M

P-X

Dormant halide capped chain P- Growing polymer radical

X Halogen L Solubilising ligand

Figure 1 General reuction scheme for Atom Tran.$er Radical Polymerisation ( A T R P )

multidendate ligands has been r e p ~ r t e d . ~ Because ATRP is based on radical, rather than ionic, polymerisation chemistry, it has gained a deserved reputation for excellent tolerance towards protic solvents and impurities and ATRP has been exploited by many research groups to polymerise a wide range of functional monomers. Several years ago we discovered that ATRP of various hydrophilic methacrylates proceeds very rapidly in water under remarkably mild conditions. For example, monomethoxy-capped oligo(ethy1ene glycol) methacrylate [OEGMA] was polymerised to more than 95% conversion within 25 minutes at 20°C and GPC analysis indicated final polydispersities of 1.20 to 1.30. Similar results were obtained with sodium 4-vinylbenzoate [NaVBA] and 2-methacryloyloxyethyl phosphorylcholine [MPC]. Matyjaszewski and co-workers have previously reported a good correlation between the rate of ATRP and the solvent polarity, although the use of water was not in~estigated.~ We have recently obtained ESR and UV-visible absorption spectroscopy data which indicate a somewhat higher CU(II)concentration in the aqueous ATRP of OEGMA compared to bulk ATRP of the same monomer. This is consistent with the observed increase in rate of polymerisation in the former case and also accounts for the somewhat higher polydispersities, which result from the increased probability of termination. If ATRP is conducted in methanol, under the same conditions, the rate of polymerisation is slower (95% conversion required 2-5 h at 20 "C).However, for several hydrophilic monomers such as 2-hydroxyethyl methacrylate [HEMA] and glycerol monomethacrylate [GMA], methanolic ATRP is the preferred method for optimal living character since narrower polydispersities (typically 1.10 to 1.20)and better blocking efficiencies are generally obtained. Compared to other living radical polymerisation techniques, ATRP offers two important advantages. Firstly, the synthesis of well-defined macro-initiators is facile and allows the preparation of a range of new diblock copolymers. Secondly, the presence of water (or methanol) has a remarkable accelerating effect on the

23

S.P. Arrnes et al.

rate of ATRP of hydrophilic monomers, allowing high conversions to be achieved rapidly even under mild conditions.6 Herein we summarise our recent progress in the exploitation of ATRP for the synthesis of controlled-structure block copolymer surfactants and dispersants.

2 Experimental We have recently evaluated the ATRP of a wide range of hydrophilic monomers such as 2-sulfatoethyl methacrylate (SEM), sodium 4-vinylbenzoate (NaVBA), sodium methacrylate (NaMAA), 2-(dimethy1amino)ethyl methacrylate (DMA), 2-(N-morpholino)ethyl methacrylate (MEMA), 2-(diethy1amino)ethyl methacrylate (DEA), oligo(ethy1ene glycol) methacrylate (OEGMA), 2-hydroxyethyl methacrylate (HEMA), glycerol monomethacrylate (GMA), 2-methacryloyloxyethyl phosphorylcholine (MPC), and a carboxybetaine-based methacrylate [CBMA]. Their chemical structures and literature references (which contain appropriate experimental details) are summarised in Table 1.

Table 1 Chemical structures and relevant literature references of hydrophilic monomers that are amenable to aqueouslmethanolic ATRP

SEM'

NaVBA'

NaMAA9

MEMA"

DEA"

Cationic

DMA" Non-ionic

)Joyo, OH

OEGMAi3 Betaine

MPCI6

I

HEMAI4

I

GMA'~

CBMA"

Synthesis of New Polymeric Surfactants and Dispersants

24

Monomer conversions were determined by ‘H NMR spectroscopy in all cases. Molecular weight distributions were assessed using THF G P C or aqueous GPC, respectively. In most cases silica or alumina chromatography was used to remove the spent Cu catalyst after the ATRP syntheses.

3 Results and Discussion Exploiting ATRP as an ‘enabling’ technology, we have recently synthesised a wide range of new, controlled-structure copolymers. These include: (1)branched analogues of ‘Pluronic’ non-ionic surfactants; (2) schizophrenic polymeric surfactants which can form two types of micelles in aqueous solution; (3) novel sulfate-based copolymers for use as crystal habit modifiers; (4) zwitterionic diblock copolymers, which may prove to be interesting pigment dispersants. Each of these systems is discussed in turn below.

3.1 Branched Analogues of ‘Pluronic’ Non-ionic Surfactants Poly(alky1ene oxide)-based (PEO-PPO-PEO) triblock and diblock copolymers are commercially successful, linear non-ionic surfactants which are manufactured by BASF and ICI. Over the last four decades, these block copolymers have been used as stabilisers, emulsifiers and dispersants in a wide range of applications. With the development of ATRP, it is now possible to synthesise semibranched analogues of these polymeric surfactants. In this approach, the hydrophobic PPO block remains linear and the terminal hydroxyl group(s) are esterified using an excess of 2-bromoisobutyryl bromide to produce either a monofunctional or a bifunctional macro-initiator. These macro-initiators are then used to polymerise OEGMA, which acts as the branched analogue of the PEO block (see Figures 2 and 3). A range of OEGMA-PPO diblocks and OEGMA-PPO-OEGMA triblocks have been synthesised by ATRP in alcohol/water mixtures with variable OEGMA content and relatively low polydispersities.’* We are now beginning to investigate their aqueous solution properties. Initial results suggest that these semi-branched surfactants are reasonably surface-active but form micelles less

Linear PI uron i cs PEO-PPO-PEO

Branched Pluronics OEGMA-PPO-OEGMA

Figure 2 Schematic representation of the linear and semi-branched architectures of two non-ionic polymeric surfactants

S.P. Armes et al.

25

I

0

Figure 3 Reaction scheme jor the synthesis of a semi-branched OEGMA-PPO diblock copolymer by A T R P using a PPO-based macro-initiator

readily than their linear counter-parts. Presumably, this is due to the poor packing efficiency of the branched OEGMA chains within the micelle corona. It remains to be seen whether these semi-branched surfactants offer any useful advantages over the conventional linear surfactants. We are also evaluating the possibility of preparing fully-branched analogues. In principle, the linear PPO block can be replaced with oligo(propy1ene oxide) monomethacrylate (OPOMA), but in practice we and others” have experienced difficulties in polymerising OPOMA with sufficient control by ATRP. To date, we have achieved high conversions but poor control over the molecular weight distribution; in most cases G P C analysis indicates multimodal traces.

3.2 Schizophrenic Polymeric Surfactants It is well known that AB diblock copolymers form micelles in solvents that are selective for one of the blocks. By varying the nature of the solvent, it is also possible to form micelles with the A block in the core or with the B block in the core. However, we have recently demonstrated that certain hydrophilic AB diblock copolymers can form either A-core micelles or B-core micelles in aqueous media.20 2 2 In the original example, both blocks were based on tertiary amine methacrylates and the diblock copolymer was prepared by group transfer polymerisation, a special type of anionic polymerisation which is particularly

26

Synthesis qf New Polymeric Surfactants and Dispersants

well-suited for methacrylic monomers. More recently, we have prepared a poly[propylene oxide-block-2-(diethylamino)ethyl methacrylate] (PPO-DEA) diblock copolymer by alcoholic ATRP using a macro-initiator strategy (see Figure 4). The PPO block is temperature-sensitive, with an LCST (cloud point) of around 15 "C, whereas the DEA block is pH-responsive. At pH 6.5 and 5 "C, both blocks are solvated and the copolymer is molecularly dissolved in aqueous solution. On adjusting the solution pH to pH 8.5, the DEA block becomes deprotonated and hence hydrophobic, leading to the formation of DEA-core micelles at 5 "C.On the other hand, if the pH is held constant at pH 6.5 and the temperature is raised above the LCST of the PPO block, PPO-core micelles are obtained with the weakly cationic DEA chains forming the micelle coronas. Since these new diblock copolymers can exist in two micellar states we have christened them 'schizophrenic' copolymers.'*

3.3 Synthesis of Sulfate-based Block Copolymers as Crystal Habit Modifiers for Bas04 We have recently synthesised a range of PEG-SEM diblock copolymers. PEG macro-initiators were used for the ATRP of SEM and the molecular weights of the PEG and SEM blocks were varied independently. In collaboration with Meldrum's group at QMC, these copolymers were evaluated as crystal habit modifiers for BaSO,, which was prepared in situ by the addition of a soluble barium salt to an aqueous solution of (NH,),SO,. In the absence of PEG-SEM copolymer, the precipitated BaS0, particles had a polydisperse, micrometresized platelet morphology (see Figure 5a). In the presence of a PEG,,-SEMZ3 diblock copolymer, egg-shaped particles were obtained (see Figure 5b). Presumably the sulfate groups in the SEM blocks interact with the crystal lattice of the

PPO-core micelles at 40-70 "C and pH 6.5

'

DEA-core micelles at 5 "Cand pH 8.5

M,=9,800 M,M, = 1.20 Soluble at 5 "Cand pH 6.5

Figure 4 Summary of aqueous solution conditionsfor the two micellar forms ofthe 'schizophrenic' PPO,,-DEA,, diblock copolymer

27

S.P. Armes et al. (4

Figure 5 Scanning electron micrographs of (a) BaS04 particles prepared in the absence of any copolymer (control experiment) and (b) BaS04 particles prepared in the presence of a PEO,,-SEM,, diblock copolymer synthesised by ATRP. Note the prcfound change in particle morphology due to the interaction of the sulfate-based block with the BaSO, crystal lattice

precipitating BaSO, and the PEG blocks act as a steric stabiliser, leading to a profound change in the particle morphology. Control experiments with PEG and SEM homopolymers confirm that the diblock copolymer architecture is essential for effective crystal habit modification. Further characterisation of these egg-shaped particles using thermogravimetry, FT-IR spectroscopy and X-ray diffraction is in progress.23 Similar crystal habit modification effects have been observed by Antonietti’s g r o ~ pfor ~ a~ range . ~ ~of carboxylate- and phosphonate-based copolymers; the advantage in the present study is the relative ease with which well-defined PEG-SEM diblock copolymers can be synthesised.

3.4 Synthesis of Zwitterionic Diblock Copolymers There has been relatively little work on zwitterionic diblock copolymers, since the synthesis of these materials normally require protecting group chemistry and anionic polymerisation technique^.^ However, it has been confirmed that these materials exhibit rich and complex phase behaviour in aqueous solution, including reversible precipitation at a certain critical pH, where the anionic charge just balances the cationic charge, i.e. the isoelectric point.26-28Moreover, in collaboration with a group at Akzo Nobel, Creutz and Jerome have that a zwitterionic diblock copolymer based on methacrylic acid [MAA] and 2-(dimethy1amino)ethyl methacrylate [DMA] appears to act as a ubiquitous dispersant for a range of inorganic and organic pigments with both anionic and cationic surface charge. In principle, aqueous ATRP offers the tantalising possibility of the direct synthesis of reasonably well-defined zwitterionic block copolymers in water without recourse to protecting group chemistry. However, ATRP in acidic media is generally unprofitable, hence the (co)polymerisation of acidic monomers such as methacrylic acid or 4-vinylbenzoic acid must be carried out in weakly alkaline solution, i.e. the monomer should be in its anionic carboxylate 632

-

28

Synthesis of New Polymeric Surfactants and Dispersants

I+ + + + + - - - I+k + - -----

Soluble since no iep Insoluble at end of polymerisation as iep is reached

+ + +

1

1

Insoluble during polymerisation when anionic charge = cationic charge (iep)

Figure 6 Schematic representation of the problems that can be encountered during the synthesis ofa zwitterionic dihlock copolymer by aqueous A T R P . 1 represents the initiator fragment and represents the block junction

form. Thus if quaternised monomers are used to synthesise zwitterionic block copolymers and the shorter block is prepared first, there is an obvious danger of passing through the isoelectric point during synthesis, leading to premature precipitation (see Figure 6). We have some evidence that this theoretical problem is a genuine limitation in the case of a quaternised styrenic monomer which is block copolymerised with NaVBA.30 This problem can be circumvented in two ways. Firstly, the polymerisation sequence can be simply reversed so that the longer block is synthesised first. If this is the quaternised block, the resulting copolymer cannot exhibit an isoelectric point because the major block is permanently cationic, thus no charge compensation can occur. On the other hand, if the longer block is anionic, then addition of HCl will protonate the acidic monomer residues and at some point an isoelectric point will be attained (unless the acidic block is strongly acidic, e.g. 4-styrenesulfonic acid). Secondly, the quaternised monomer may be replaced with a weakly basic monomer such as MEMA, which exists in its neutral, non-protonated form in alkaline media. Thus the desired zwitterionic block copolymer is prepared in its anionic/neutral form so that no isoelectric point is encountered during the copolymer synthesis. Afterwards, the solution pH can be adjusted to the isoelectric point by the addition of acid to protonate the weakly basic MEMA residues and precipitate the copolymer, which might be a useful alternative approach to column chromatography for the efficient removal of the ATRP catalyst.

4 Conclusions ATRP is a powerful synthetic tool for the synthesis of low molecular weight (Dp < 100-200), controlled-structure hydrophilic block copolymers. Compared to other living radical polymerisation chemistries such as RAFT, ATRP offers two advantages: (1)facile synthesis of a range of well-defined macro-initiators for the preparation of novel diblock copolymers; (2) much more rapid polymerisations under mild conditions in the presence of water. In many cases these new copolymers have tuneable surface activity (i.e. they are stimuli-responsive) and exhibit reversible micellisation behaviour. Unique materials such as new ‘schizo-

S.P. Armes et al.

29

phrenic’ diblock copolymers or sulfate-based copolymers for crystal habit modification are readily prepared by ATRP. Finally, zwitterionic diblock copolymers can be synthesised directly by aqueous ATRP without recourse to protecting group chemistry and may have interesting properties as ubiquitous pigment dispersants. Two major challenges remain: (1)the more efficient and cost-effective removal (and preferably recycling) of the ATRP catalyst and (2) the extension of aqueous/methanolic ATRP to include other classes of monomers such as acrylates and (meth)acrylamides.

Acknowledgements The following sponsors are thanked for their continued support of the watersoluble polymers research programme at Sussex: EPSRC, BBSRC, ICI Paints, Unilever, Tot alFinaElf, GlaxoSmit hKline, Syngen t a, Avecia, Biocompa t ibles and Laporte Performance Chemicals.

References 1. J.S. Wang and K. Matyjaszewski, J . Am. Chem. Soc., 1995,117,5614. 2. M. Kato, M. Kamigaito, M. Sawamoto and T. Higashimura, Macromolecules, 1995, 28, 1721. 3. T.E. Patten, J. Xia, T. Abernathy and K. Matyjaszewski, Science, 1996,272,866. 4. J.H. Xia, S.G. Gaynor and K. Matyjaszewski, Macromolecules, 1998,31, 5958. 5. K. Matyjaszewski, Y. Nakagawa and C.B. Jasieczek, Macromolecules, 1998,31,1535. 6. X.S. Wang, S.F. Lascelles, R.A. Jackson and S.P. Armes, Chem. Commun., 1999,1817. 7. K.L. Robinson, D.Phi1; University of Sussex, UK, 2001. 8. X.S. Wang, R.A. Jackson and S.P. Armes, Macromolecules, 2000,33,255. 9. E.J. Ashford, V. Naldi, R. O’Dell, N.C. Billingham and S.P. Armes, Chem. Commun., 1999,1285. 10. F.Q. Zeng, Y.Q. Shen, S.P. Zhu and R. Pelton, J . Polym. Sci., Polym. Chem., 2000,38, 3821. 11. F.L.G. Malet, N.C. Billingham and S.P. Arrnes, ACS Polym. Prep., 2000,41(2), 1811. 12. S.Y. Liu, N.C. Billingham and S.P. Armes, Angew. Chem., Int. Ed., 2001,40,2328. 13. X.S. Wang and S.P. Armes, Macromolecules, 2000,33,6640. 14. K.L. Robinson, M.A. Khan, M.V. de Paz Bafiez, X.S. Wang and S.P. Armes, Macromolecules, 2001,34,3155. 15. M. Save, J.V.M. Weaver and S.P. Armes, Macromolecules, 2002,35,1152. 16. E.J. Lobb, I. Ma, N.C. Billingham, S.P. Armes and A.L. Lewis, J . Am. Chem. Soc., 2001,123(32), 7913. 17. I. Ma, E.J. Lobb, S.P. Armes and N.C. Billingham, manuscript in preparation, 2001. 18. K.L. Robinson, M.V. de Paz Baiiez, X.S. Wang and S.P. Armes, Macromolecules, 2001,34(17), 5799. 19. B.L. Sadicoff, A.E. Acar and L.J. Mathias, ACS Polym. P r e p . , 2000,41(2), 128. 20. V. Butiin, N.C. Billingham and S.P. Armes, J . Am. Chem. Soc., 1998,120,12135. 21. V. Butun, N.C. Billingham and S.P. Armes, J . Am. Chem. Soc., 1998,120,11818. 22. V. Butiin, N.C. Billingham and S.P. Armes, Macromolecules, 2001,34, 1148. 23. K.L. Robinson, J.V.M. Weaver, S.P. Armes, E. Diaz Marti and F.C. Meldrum, J .

30

Synthesis of New Polymeric Surfactants and Dispersants

Muter. Chern., 2002,12(4), 890. L. Qi, H. Colfen and M. Antonietti, Chern. Muter., 2000, 12, 2392. L. Qi, H. Colfen and M. Antonietti, Anyew. Chem., Int. Ed., 2000,39,604. A.B. Lowe, N.C. Billingham and S.P. Armes, Macromolecules, 1998,31, 5991. S. Creutz and R. Jerome, Langmuir, 1999,15,7145. S. Creutz and R. Jerome, Prog. Org. Coatings, 2000,40,21. S. Creutz, R. Jerome, G.M.P. Kaptijn, A.W. van der Werf and J.M. Akkerman, J . Coatings Tech., 1998, 70,41. 30. S.A. Furlong, D.Phil., University of Sussex, UK, 2001.

24. 25. 26. 27. 28. 29.

Role of Colloidal Interactions in Determining Rheology D.M. Heyes and J.F.M. Lodge DEPARTMENT OF CHEMISTRY, UNIVERSITY OF SURREY, GUILDFORD, GU2 7XH, UK

1 Introduction Colloidal liquids are widespread in the natural world and in manufactured processes and products (e.g. foods, paints and other coatings, agrochemicals, ceramics and polymers). The macroscopic behaviour of these liquids is largely governed by the colloidal particle interactions and it is therefore on this scale that a true understanding of these materials must be sought. The relationships between the physical properties of colloidal liquids and the interactions between the particles are still not well understood, especially when there are strong attractive interactions between the particles. The complexity of these systems, together with the added complication of hydrodynamic interactions between the particles, means that analytic treatments of their dynamical and physical properties are highly approximate. There has been only limited progress in quantifying the effects of chemical sensitivity on the physical properties. Situations of practical importance also often employ high concentrations, sometimes up to closepacking, in which particle ‘crowding’ factors are also important. Molecular simulation is a useful quasi-experimental tool for exploring these issues. Molecular simulation and perturbation theory have been used in this study to achieve an improved understanding of the relationship between the particle interactions and the molecular relaxation processes occurring in these liquids, and in particular their impact on the linear viscoelasticity. We have considered some basic interaction types which are found in colloid systems.

2 Exploring the Potentials The systems considered included a steeply repulsive potential, SRP, between the particles with n arbitrary:

31

Role of Colloidal Interactions in Determining Rheology

32

These are purely repulsive particles. An attractive potential of the Lennard-Jones type, with n arbitrary: V(r)= 4 & [ ( y r

-

(;>"I

was also considered. These particles have an attraction at intermediate and long range, which enables the particles to aggregate and even form network or gel-like states at low concentrations.

2.1 Near-hard Sphere Fluids - a Step Towards Stabilised Colloids The steeply repulsive particles, SRP, whose interaction potential is given in Equation (l), offer an extremely useful starting point for understanding the rheology of colloidal liquids because of the versatility of this potential in representing a wide range of stabilised systems. Equation (1) has the useful feature that in the large n limit ( n + a)we approach the hard sphere potential. Colloid scientists talk of 'hard sphere' colloids, and this potential can explore both this limit and much softer repulsions (down to n = 1, the Coulomb interaction, in principle) just by adjusting a single parameter (n). Using molecular dynamics computer simulation and statistical mechanics, some fundamental characteristics of the viscoelasticity of these fluids have been The shear stress relaxation function, C(t)(the response of the shear stress at time t after a step in shear strain at time t = 0) is the key quantity in defining viscoelastic behaviour. It was shown that for the SRP fluids (without the solvent) there is an accurate expansion of C(t)about t = 0 for n large. For C(t)(normalised so that C(0) = 1) we find, C(t) = 1 - X2

+ . . . O(x4)

(3)

where x = n ( k g T / ~ ) l / ~ ( ~ / r n (and r ~ ) lrn/ ~ist the mass of the SRP particle. The parameter, x, is a non-dimensional time that includes the temperature and the potential stiffness, n. Note that the stress relaxation function is independent of density, which is not surprising as in the large n limit, the short time decay of C(t) is dominated by binary collisions at all densities, even near close packing. (Three and higher body effects are required to introduce a density sensitivity.) It is tempting to extrapolate this series by performing the following closure, which might be applicable at all times: C(t) =

1 ~

1

fX2

=

1 - x2 + x4

-

O(X6)

(4)

The function fits the C(t) data very well.'12 Note that the n-dependence of the reduced time parameter x means that the relaxation time, z (which is the area under C(t)) decreases as n-'. The dependence of the viscosity, y, with n is much

D.M. Heyes and J.F.M. Lodge

33

weaker however as y = G,z, where G, is the infinite frequency elastic modulus, which is proportional to n for large n. The two n dependencies do not exactly cancel out in practice, as C(t)has contributions from cage effects at long time which cause deviations in the form of C(t)from the analytic expression given in Equation (4).Nevertheless, the n-dependence of the viscosity is much less that the G, and z separately. In Figure 1 we show an example of the short-time scaling of C(t) when plotted against nt, with n = 144 and 1152 as examples, and when compared also with Equation (4). The effective hard-sphere packing fraction is 0.45 and the reduced temperature is kT/c = 1 in this figure. One conclusion from this study is that although the hard-sphere fluid has been very successful as a reference fluid, for example, in developing analytical equations of state, it is unrealistic in representing the dynamical relaxation processes in real systems, even with very steeply repulsive potentials. Owing to the discontinuity in the hard-sphere potential, this fluid, in fact, is not a good reference fluid for the short time (fast or ‘p)viscoelastic relaxation aspects of rheology.

2.2 Gel Formation and Ageing - the Role of Particle Attraction Strength and Range Despite its importance in many consumer products, the physical origins of the gel state, and its dependence on the chemical forces between the particles, is still quite poorly understood. Various approximate theoretical models exist. For example, the fractal-based approach has had considerable success in fitting to a wide range of experimental data, but only over certain parameter ranges - for

0

1

3

2

4

5

nt Figure 1 The shear stress relaxation function, C(t), obtained from a molecular dynamics simulation of 500 SRP spheres at a reduced temperature of 1 .O and eflective volume fraction of 0.45. Note that n = 144 and 1152 (from Equation (1)) cases are superimposable with the analytic function of Equation (4) (‘Algebraic’ on the figure) for short times, t (or nt here)

34

Role of’ Colloidal Interactions in Determining Rheology

strong particle attractions and at particle densities close to the percolation threshold. The fractal model for rheology makes rather sweeping assumptions about particle and cluster dynamics, and is therefore only applicable in the time and space ‘scale-invariant’ regimes close in time to the percolation threshold itself. The early success of this approach has in many ways stifled further theoretical work in this area, even though there is no specific chemical focus in these treatments. One can view the gel state as a ‘quench’ into the two phase region of the phase diagram (‘vapour-solid’ or ‘vapour-liquid’ states of the associating particles) which leads to the formation of a percolating network, substantial condensation of the assembly, and local structure evolution can take place, even for a system with a fractal dimension of ca. 3. The ‘constituency’ of the gel depends on a range of factors, such as the strength and range of the attraction between the particles, as well as their concentration. Brownian dynamics, BD, simulations were carried out on model colloidal liquids interacting with the potential given in Equation (2).3 The structural evolution and rheology of these model attractive spherical colloidal particles were followed as they self-assembled into long-range networks. The procedure used was to ‘quench’ the particles from a supercritical state point into the vapour-liquid or vapour-solid parts of their phase diagrams. The solids volume or packing fractions, #, were in the range 0.05-0.20. The interactions between the model colloidal particles had y1 = 6, 12 and 18 in Equation (2),giving rise to 12:6, 24:12 and 36:18 potentials, respectively. Along this series, the attractive part of the potential becomes shorter ranged. These systems developed a gel-like morphology during the simulations, with the aggregate morphology and rheology being sensitive to the range of the attractive part of the potential and the position in the phase diagram of the quench. A typical example is shown in Figure 2, for the long-range 12:6 potential, which self-assembled on intermediate timescales to compact structures with thick filaments. The systems generated using the shorter-ranged 24:12 and 36:18 potentials persisted in a more diffuse network for the duration of the simulations and evolved more slowly with time. The phase separation also becomes apparent in a time-evolving pair radial distribution function, g(r), an example of which is shown in Figure 3 for a quenched 24:12 system. With time after the quench, the y(r) shows the appearance of a second and even third coordination shell. Notice that the second coordination shell peak is actually formed from two overlapping peaks, which is a signature typical of the glassy state. The condensation of the particles with time is also manifest in the average interaction energy per particle, u, which is shown in Figure 4 for various packing fractions (0.05 to 0.20) as given on the figure. The denser systems form more compact structures and therefore develop the most negative energy values for u with time. The rheology of many of the systems displayed gel-like viscoelastic features, especially for the long-range attractive interaction potentials, which manifested a non-zero plateau in the shear stress relaxation function, C,(t), the so-called equilibrium modulus, G,,, which has been considered to be a useful indicator of the presence of a gel. The infinite frequency shear rigidity modulus, G, was extremely sensitive to the form of the potential. Despite being the most short-

D.M. Heyes and J.F.M. Lodge

35

Figure 2 Snapshot picture depicting the aggregate structure at tap2Do= 164 (where a is the particle radius and Do is the selj-difluusion coeficient of the particles at injinite dilution). There are 864 particles in the simulation with particles that interact with the 12:6 potential at the state point, (b = 0.2 and T* = 0.3. Notice the,filarnentary nature of the agglomeration of the particles 25

20

15

n k W

b10

10

5

n 0

1

2

3

4

5

6

7

8

r Figure 3 The g(r) at various tap2DD, (shown on thejgure) for the 24:12 interaction potential, with (b = 0.1 and T* = 0.3

Role of Colloidal Interactions in Determining Rheology

36 -1

12-6 interaction potential

0

T* = 0.3

-2

A 5 V

0

#J

= 0.05

0

#J

= 0.10

@ = 0.16

-3 A

@

= 0.20

-4

-5

-6

40

0

80

200

160

120

tam2Do Figure 4 Time evolution of the average interaction energy, u, for the 12:6 interaction at T" = 0.3.fordiflerent volume fractions, which are given on theJigure

l6

I

36:18 interaction potential T* = 0.3

$I = 0.20

to(a'lDd

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

td2Do Figure 5 Time evolution of the shear stress relaxationfunction, C(t),for the 36:18 potential at 4 = 0.2 and T* = 0.3. The waiting times are tw = 3 and 162for the two curves

lived, the 12: 6 potential systems gave the most pronounced gel-like rheological features. In common with glasses, the dynamics of particle gels can be strongly dependent on its history of formation. One finds that the relaxation dynamics become increasingly slower with 'age' or 'waiting' time from the quench, t,. The stress relaxation function now depends on two times, C(t,,t t,). The larger t,, the

+

D.M. Heyes und J.F.M. Lodge

37

slower the function decays up to times of order 2t,. In fact, for Lennard-Jones glasses the long time behaviour of the function can be scaled quite well onto a One would expect the single curve using the reduced time parameter nature of the aging process in transient gels to be different from the glassy case because of the accompanying long-range morphological changes, which are additional important factors in the ageing process. Parallel ageing takes place on a range of lengthscales, much larger than in the case of glasses. We found this ageing effect occurred in our systems. It was manifest as a ‘plateau’ in C(t) at intermediate times, which grew in height and duration with t,. An example of this behaviour is given in Figure 5 for the 36:18 potential system at a packing fraction of 0.20 and reduced temperature of 0.3. Note that after a time of 162 reduced time units after the quench, the shear stress relaxation function has developed an intermediate plateau, characteristic of ageing systems. t/tw.415

3 Conclusions The research carried out on these very simple potential forms has revealed that they capture many of the physical/rheological features present in real systems. By focusing on such potentials, which have one or two adjustable parameters one can gain insights into generic effects that span many examples of chemical system. These potentials are also, on occasions (e.g. the potential given in Equation (1)) amenable to simple analytic treatments.

Acknowledgements The authors would like to thank the Engineering and Physical Sciences Research Council of Great Britain (EPSRC) for funding this work.

References 1. J.G. Powles and D.M. Heyes, Mol. Phys., 2000,98,917. 2. J.G. Powles, G. Rickayzen and D.M. Heyes, Proc. Roy. SOC.: Series A, 1999,455,3725. 3. J.F.M. Lodge and D.M. Heyes, Phys. Chem. Chem. Phys., 1999,1,2119. 4. J.-L. Barrat and W. Kob, Europhys. Lett., 1999,46,637. 5. J.-L. Barrat and W. Kob, J . Phys.: Condens. Mutter, 1999,11, A247.

New Polymeric Dispersants for Very Fine Calcium Carbonate Slurries Y. Duccini and A. DuFour ROHM AND HAAS FRANCE SAS, EUROPEAN LABORATORIES, SOPHIA ANTIPOLIS, 06560 VALBONNE, FRANCE

1 Introduction Calcium carbonate is the most widely used filler mineral in the world. Its success is mainly due to its availability and also its suitability for a large range of applications. Many industries use calcium carbonate: Paper industry (as filler and for coating) Paint industry (as filler and extender) Plastic industry (as filler, but bringing interesting properties) Sealant and adhesive industry (as filler) Each application requires very specific product characteristics in terms of: Chemical purity Particle size distribution Particle shape and surface area Whiteness Rheological behaviour There are two sources of calcium carbonate, ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC). Ground calcium carbonate, extracted from the earth, is present in practically every country in the world in varying quantities in the form of limestone, marble, dolomite or chalk. Following the extraction, GCC needs to be ground. Dry grinding, the cheaper alternative, is often limited to a minimum particle size of 2--3 microns. Wet grinding, more expensive, is used for fine and ultra fine material or when the final product must be a slurry (paper or paint application). Precipitated calcium carbonate is produced by chemical reaction between 38

39

Y. Duccini and A . DuFour

quick lime and carbon dioxide. By changing the reaction conditions, numerous types of PCCs can be produced, differing in terms of particle size, particle shape, crystal type, etc. PCC is generally not ground. It can be sold as dry powder or as slurry. When calcium carbonate (GCC or PCC) is sold in a liquid form, the concentration of solids in the slurry is maximised to minimise transportation of water and to reduce energy consumption for drying in some applications. The way to maximise the solids content of the slurry while keeping a reasonable viscosity is through the use of a dispersant. The dispersant helps maintain fine solid particles in a state of suspension, thus minimising their agglomeration or settling. The dispersant is generally a low molecular weight acrylic based polymer. For most applications, cost of dispersant is critical. The paper industry requires the finest grade of calcium carbonate for coating of the paper sheet. The brightness of the paper sheet improves with increasing fineness of calcium carbonate in the coating colour. In addition, coating machines operating at very high speed demand a very good rheological behaviour of the coating colour, primarily low viscosity at high speed. These specific requirements of a calcium carbonate slurry used for paper coating have necessitated the development of new dispersants that allow the preparation of ultra fine calcium carbonate slurries at high solids concentration that provide good rheological behaviour in coating. This article studies some of the polymer characteristics influencing the slurry dispersion and describes a new class of acrylic based polymer showing promising dispersing properties for ultra fine calcium carbonate slurries.

2 Acrylic Based Dispersants Manufacturing These polymers are synthesised in water by free radical polymerisation. In the reaction, many components are involved, principally: The monomers (M, M', etc.) that will create the backbone of the polymer. The initiator (R-R) that will provide free radicals to start the polymerisation reaction. The chain transfer agent (X-A) that will regulate the molecular weight of the polymer. The process is schematically the following when using only one monomer (acrylic acid): Initiation: R-R + 2 R' R*+M-+RM* Propagation:

RM'

+ M -+ RM,'

40

New Polymeric Dispersants for Very Fine Calcium Carbonate Slurries

Regulation: RM,'

+ X-A + RM,-X + A' A*+M+AM' AM' + M AM,' --+

Termination: RMp' + RMq'+RM(P+q)R In the industry, the polymerisation of more than one monomer gives a random copolymer, monomers being distributed along the chain without any order. Many different chain transfer agents have been used to produce such polymers. These are mercaptans (e.g. mercaptopropionic acid), alcohols (e.g. isopropanol), or inorganic compounds (e.g. bisulfite, hypophosphite, etc.). The chain transfer agent is critical for production of low molecular weight polymers and can also influence performance properties.

3 Acrylic Based Dispersant Characterisation Acrylic based polymers can be characterised by: Molecular weight (M, and M,) Polydispersity (defined as M,/M,) Monomer type Monomer ratio End group Neutralising agent To that, we have to add the concentration of salts of the resulting aqueous solution of polymer. These salts come from the residual polymerisation auxiliaries and the neutralising agents.

4 Influence of Polymers Characteristics on Slurry Dispersion A good dispersant must allow the preparation of slurries that provide the highest solids content with the lowest (or at least acceptable) viscosity. For paper coating the paper industry increasingly requires very fine particle size and high solids calcium carbonate slurries. Today, it is normal to prepare slurries at 78% solids with 90% (and even more) particles less than 2 microns. Fine particle size calcium carbonate provides high brightness of the paper sheet, while a high solids slurry allows improved paper coating quality (less mottling, better gloss, better drying, etc.). These very fine particle size slurries require the use of improved dispersants. The previously mentioned polymer characteristics play an important role on dispersion efficacy. Depending on the type of mineral and its particle size, there will be an optimum molecular weight allowing a good

Y. Duccini and A . DuFour

41

dispersion. We know that commercial polymers do not have a single molecular weight but have a range around an average value. This range is evaluated by the polydispersity. The lower the polydispersity, the narrower the range. Low polydispersity is beneficial since there are fewer low molecular weight polymeric chains that are inefficient dispersants and fewer high molecular weight polymeric chains that can act as flocculants. The monomer type and ratio change the adsorption properties of the polymer on the calcium carbonate particles. The type of end group (which depends on the polymerisation process) can also influence adsorption characteristics. The neutralising agent as well as the salt content influences the slurry ageing. A good dispersing agent must maintain constant viscosity over time. We know that a lower monovalent cation content improves slurry stability. The influence of molecular weight, polymerisation process, monomer type and neutralisation have been studied.

5 Calcium Carbonate Tested Three different sorts of calcium carbonate have been used for tests, one GCC and two PCCS. The first PCC has a particle size of 70% < 2 microns. The slurry was prepared at 75% solids, and the dispersant concentration was 0.80% (as polymer solids/dry CaC0,). The second PCC is finer, its particle size is 95% < 2 microns. Slurry solids was also 75%, and dispersant concentration was 0.80% (as polymer solids/dry CaC03). The tested GCC was ground to a particle size of 90% < 2 microns. The slurry concentration was 74.5%, and dispersant concentration was 0.73% (as polymer solidsldry CaCO,). The following measurements have been performed on PCC slurries: PH Solids Brookfield viscosity at 10,50 and 100 rpm Ageing test at T = 0, T = 24 hours and T = 48 hours On GCC slurry, we measured: PH Solids Conductivity Particle size distribution Brookfield viscosity at 10,20,50 and 100rpm Ageing test at T = 0 up to T = 5 days Brookfield viscosity measurement is a way to estimate the quality of a fresh dispersion. The ageing test controls the evolution of the viscosity over time. It is

42

New Polymeric Dispersants,for Very Fine Calcium Carbonate Slurries

important to maintain low viscosity for several days, in order to keep the slurry workable. The ageing test is particularly important for fine particle size slurries since they tend to gel when an inappropriate dispersant has been used. The ageing test measures the torque necessary to induce the rotation of a special spindle in the slurry. The measure must be done before the first rotation is completed in order to prevent destruction of the gel network. We measure gel after 25 seconds using a Brookfield viscometer with a special spindle (Helipath type), rotating at 1 rpm. Particle size distribution was measured with Coulter LS 230 laser equipment. Solids were determined with a drying scale. For each slurry, solids were corrected ( & 0.1YO)prior to viscosity measurements. For GCC grinding, the tolerance on particle size distribution was & 1% (89-9170 < 2 microns).

6 Influence of Polymerisation Process Rohm and Haas developed a new polymerisation process based on the use of a new chain transfer agent (process C). Polymers made with this new process showed very good dispersing activity. Polymers prepared by current industrial processes have been compared with this new process in the dispersion of GCC and PCCs (Tables 1-3). These current processes use as chain transfer agent inorganic salts (processes A and B) or alcohol (process D).

Table 1 Fine PCC (95% < 2 microns), slurry solids 75%, dispersant 0.80% (as polymer solidsldry powder)

Surnple

Process A B C

D

Viscosity Brookjield 100 rpm (mPas)

Ageing After 48 hour (/lo6mPas)

1072 1158 614 582

20.9 15.7 10.3 15.2

Table 2 Coarser PCC (70% < 2 microns), slurry solids 75%, dispersant 0.80% (us polymer solidsldry powder). Process A was not tested with this PCC

Sample

Process

Viscosity Brookjeld 100 rpm (mPas)

1 2 3

B C D

604 444 482

Ageing After 48 hour (/lo6mPas)

9.1 5.5 9.2

Y. Duccini and A . DuFour

43

Table 3 GCC, slurry solids 7 4 3 4 , dispersant 0.73% (as polymer solidsldry powder), grinding at 90% < 2 microns ~~

Sample

~~

~

Process

Viscosity Brookjield 100 rpm (mPas)

Ageing After 5 days (/1O6 mPas)

A B C D

318 242 166 160

138 138 39 37

The average molecular weight (M,) of the tested polymers is 4000 and polydispersity is about 1.2. From the three series of tests, we can rank the polymer processes in term of dispersing efficacy for calcium carbonate as follows:

Since we tested industrial polymer samples which may have differences apart from the chain transfer agent, we cannot conclude that the difference in efficiency is only due to the chain transfer agent.

7 Influence of Dispersant Molecular Weight The influence of the molecular weight of the dispersant on the quality of the slurry dispersion has been studied on both PCCs. The dispersants ranged in molecular weight from 2000 to 5800 (according to our internal standards) made by the process C. Results are summarised in Tables 4 and 5, below. We clearly see that for both PCC qualities, the optimum molecular weight for the dispersant is around 5000. Ageing is better for the coarser PCC grade. The observation that the finer the calcium carbonate particle size, the higher the tendency to gel is likely due to fine particles having greater surface area, thus having inadequate dispersant to cover the surface and provide particle stability. The ageing test indicates that the higher the molecular weight, the worse the ageing behaviour. In both cases, the lower molecular weight dispersant gives the least viscosity increase. The long polymeric chains probably create a threedimensional network by particle bridging which cause some slurry gelation. There will be a compromise between the initial viscosity and the ageing behaviour in the choice of the best dispersant.

8 Influence of Monomers The monomers constituting the backbone of the polymer influence the adsorption on particles and thus dispersion. Two polymers, prepared by the process C,

44

New Polymeric Dispersants for Very Fine Calcium Carbonate Slurries

Table 4 Fine PCC (95% < 2 microns), slurry solids 7 7 4 , dispersant 0.80% (as polymer so 1idsldry powder) ~

Sample

~~

Process

Mw

Viscosity Brookfield 100 rpm (mPas)

Ageing After 48 hours (/lo6mPas)

2000 3600 4880 5170 5800

1086 614 512 394 430

8.5 10.3 17.9 19.3 23.4

~

C C C C C

Table 5 Courser PCC (70% < 2 microns), slurry solids 75%, dispersant 0.80% as polymer solidsjdry powder)

Sample

Process

1 2

C

C

M'

3600 5170

Viscosity Brookjield 100 rpm (mPas)

Ageing After 48 hours (/lo6mPas)

444 344

5.5 11.8

Table 6 Efect of monomer composition on.fine PCC dispersion

Sample

Process

M w

Viscosity Brookfield 100 rpm (mPas)

1 2

AA AA/X

2000 2000

1086 523

Ageing After 48 hours (/lo6mPas) 8.5 8.7

differing only by their monomer composition were tested on fine PCC dispersion (Table 6). We see that the second monomer greatly improves the dispersing effect. Here again we can note that a low molecular weight polymer gives a good ageing behaviour.

9 Influence of Polymer Neutralisation The influence of salt content in the polymer solution, and particularly monovalent salts used to neutralise the polymer, were also studied. Fully neutralised and 60% neutralised polymers from the different processes were tested as dispersants for GCC grinding. Conditions were 74.5% solids slurry, 0.73% dispersant (as solids/dry powder) and 90% particle < 2 microns. Results are summarised in the Table 7. By using partially neutralised polymers, we increase the adsorption of the polymer on the calcium carbonate particle surface. More polymer being adsor-

Y. Duccini and A . DuFour

45

Table 7 Eflect of dispersant neutralisation on GCC dispersion

Sample

Process

% Neutral

Viscosity Brookjield 100 rpm (mPas)

A A B B C C D D

100 60 100 60 100 60 100 60

318 182 242 204 166 158 160 204

Ageing After 5 days (f1 O6 mPas)

138 11.4 138 12.8 39 8 37 9

bed and less remaining in solution, dispersion is improved and gelation is reduced. Dispersion is particularly improved by using partially neutralised polymers with standard dispersants like processes A and B since, due to their relatively poor efficiency, adsorption was far from complete with 100% neutralised dispersant amount. With processes C and D,dispersion improved very little although gelation tendency was reduced.

10 Special Purpose Dispersant We developed another dispersant presenting properties for a special application. In some cases when an ultra fine calcium carbonate slurry is spray dried, a re-agglomeration of the powder often occurs, due to high water adsorption. This powder aggregation is detrimental to the quality of the final product. Hygroscopicity tests were performed on slurries prepared with different dispersants after drying. Figure 1 shows that the water re-adsorption after drying is significantly reduced with the polymer called Acusol460 N. Acusol460 N is an acrylic based copolymer containing a non-ionic monomer bringing a hydrophobic character which decreases the water re-adsorption of the spray dried powder.

11 Conclusion With the paper industry requiring more and more sophisticated products from mineral producers and particularly finer and finer calcium carbonate particle size slurries, the need to improve dispersants is becoming a reality. The work described in this paper is the beginning of research on improved dispersants for preparation of PCC and GCC slurries and also other minerals. The new route (process C) for manufacturing acrylic based polymers seems very promising. Polymers prepared by this way attain the efficiency of the best dispersants on the market. In the future, we will explore this route, improving parameters which are known to influence dispersant efficiency such as molecular

New Polymeric Dispersants for Very Fine Calcium Carbonate Slurries

46 1.4 1

# #

0

2

4

6

I

8

--*-

AcurnerSBOO

10

12

14

16

18

20

Time (h) ACUSO~460 N

Figure 1 Water re-adsorption of dried calcium curhonate prepared with two diferent dispersants

weight, polydispersity, monomer type, monomer ratio, etc. Understanding the adsorption mechanism of the polymer on calcium carbonate particles is also something important which should help the design of an improved dispersant. Increasing the level of adsorption of the polymer on particles will greatly enhance the dispersing efficiency. Some special applications require specific dispersants. The case described in this paper shows that adding a property like hydrophobicity to a dispersant can help solve a problem in the final product. This is something we should not neglect when helping our customers to improve the quality of their products.

AMP-95TMfor Mineral Dispersion Thomas Gole ANGUS CHEMIE GMBH, A SUBSIDIARY OF THE DOW CHEMICAL COMPANY, ZEPPELINSTRABE 30, D-49479 TBBENBUREN, GERMANY

1 Introduction Dispersion is a term for systems containing various phases of at least one continuous and one finely dispersed. Referring to mineral slurries, this is typically a suspension of a mineral in water. This suspension normally contains some more additives for improved stability. One important additive in these systems is the dispersant. Interparticle forces hold the particles together and these interactions are reduced by the use of dispersants. This can be indicated by improved rheology profiles. Dispersants function through various mechanisms. For water-based systems the preferred mechanism is stabilisation by ionic repulsion. A repulsion force layer is formed around the mineral particle. To maintain the suspension stability, the thickness of this layer around each particle has to be increased with increasing particle size. Layer decay is more frequent with the use of small particles, which results in higher proneness to partial flocculation. Also a uniform layer is necessary for effective stabilisation of all dispersed particles. AMP-95TMhelps to achieve all these requirements. AMP-95TMis a non-polymeric charge stabiliser; because of its organic structure and low molecular weight of 89 it is often referred to as a micro dispersant (Figure 1).

2 The Concept of Charge Stabilisation Each pigment/mineral has a surface charge. This charge is dependant on various factors, among others the chemical structure of the mineral itself, the shape, geometry and particle size, but also the surface treatment. The surface charge is zero at the so-called isoelectric point (IEP) which relates to a very specific pH. The IEP of most pigments is around a pH of 3-7. A highly adsorbing alkaline agent, such and AMP (Figure 2), stabilises the charge and therefore the electro47

A M P - 9 Y Mfor Mineral Dispersion

48

/ AMP Dispersant Polyacrylate Anionic Dispersant Macro Dispersant

a

AMP Non-Polymeric Charge Stabilizer Micro Dispersant

'

Polymeric Dispersant

Figure 1 Schematic representation of polymeric and A M P dispersant mode of action

CH3

I

NH2 - C - CH2OH

I CH3 Figure 2 Molecular structure of AMP-95TM 60 40

.Q c

20

3 0

0

Y

n

Ca

3

-20 -4 0

PH

Figure 3 Plot of zeta potential vs. pH for a typical mineral suspension

static repulsion. The pH is higher than the IEP and a negative charge and potential is developed (Figure 3).

3 AMP-95TMProperties AMP'S main use as a dispersant is due to the effect of ionic repulsion. Its functionality is to shift the pigment/filler surface charges to above the isoelectric point and thereby rendering the electrostatic repulsion of the pigment particles, When two particles come close together the diffuse layers penetrate each other and thereby generate the repulsion effect. The repulsion force has to be greater than the attracting van der Waals forces to maintain dispersibility. Typically the thickness of the diffuse layer has to be increased with increasing particle size because van der Waals forces also increase. In addition, the layer decays more easily when the particle size is small, and the system is more susceptible to flocculation. Micro dispersants such as AMP are very effective in this respective area. Also, a uniform layer is necessary for effective stabilisation of all dispersed

49

Thomas Gole

particles. AMP is small enough to penetrate easily to the pigments surface and adhere strongly by adsorption/neutralising acidic moieties. It is small enough not to form double coordination, as longer molecules are able to do. The branched structure (hydrophobic side chain) assists in this. Not only, as stated above, is the electron density enhanced; the side chains also are thought to prevent intermolecular hydrogen bonding. With many other dispersants, synergistic effects are noticed.

4 AMP-95TMAdsorption and Viscosity Profile Adsorption isotherms for AMP-95TMonto kaolin, titania and calcium carbonate are shown in Figure 4. Viscosity profiles obtained when AMP is used to disperse titania and kaolin are shown in Figures 5-7. A d s o r p t i o n o f 2 - A m i n o - 2 - M ethyl-1 - P r o p a n o 1 ( A M P ) onto p i g m e n t surfaces 0.40 7

v)

B SI

035-

I

C

030-

L Q

025-

E-

*

+#l

Clay, 40% solids

+#2

Clay, 40% solids

-t D e l a m in a t e d C l a y , 4 0 % solids

Q.

d

+flutile Titanium D i o x i d e , 50% solids -Ground Calcium Carbonate, 50% solids

020-

5: 015-

000

4 0 0

10

05

15

20

25

30

w e i g h t % total A M P in d i s p e r s i o n , p e r p i g m e n t solids

Figure 4 Adsorption isotherms for AMP-95 TM onto kaolin, titania and calcium carbonate

AMP-90 for T i 0 2 Dispersion (76,5%)

fn

2

600 500

'>E 400 .8 >

A

v

300 200

I

I

I

I

50

AMP-95TMfor Mineral Dispersion

-

-

Clay #1 slurry 72% solids 0.05% AMP on solids loo0

I

Standard

aol I am

I

I

I

I

I

I

a1

1

10

100

loo0

loo00

I

Shear rate (l/s)

Figure 6 Viscosity profile for kaolin dispersed with A M P and polyacrylate

Clay #1 slurry, Hercules Viscometer data (Bob "A", 1100 rpm)

n

f 100

rn

n E 80

IStandard

0 72% solids

0.1 pph

Figure 7 Hercules viscosities for A M P and polyacrylate dispersed kaolin

Table 1 Colour details Formulation 1

PPh

No. 1 Clay CaCO, Dispersant Rheology additive Latex Starch Thickener OO / Solids

70 30 0.00- 0.60 Test 8 4 Not used 64.5-67%

Thomas Gole

2200

51

-

LUUU

0,oo

0,lO

0,30

0,20

dosage (PPh)

Figure 8 Dispersant demand curve at low shear rate (Brookjeld 30 rpm) 100 95

90 85 80 75 70 65

0,lO

0,oo

0,30

0,20

Figure 9 Dispersant demand curve at high shear rate (Hercules, bob E , 4400 rpm) 100

90 A

u)

a m E

v

8o

.-

$

70

5 a, v)

60

2 0 I 50

64

64.5

65

65.5

66

66.5

67

67.5

YO Solids

Figure 10 Hercules viscosity versus solids plot for 70/30 ClaylCaCO, colour

52

AMP-95TMfor Mineral Dispersion

5 Example of the Use of AMP-95TMin a Typical Application - Paper Coating The use of AMP as a rheology additive in paper coating is illustrated below. Table 1 gives formulation details. Figure 8 shows the colour viscosity at low shear rates and Figure 9 shows the colour viscoisty at high shear rates. The application was board basecoating at 62.5% solids. sheet coating I

- 75'

gloss

8 1 .OO 80.50

80.60

I 80.14

80.00 79.50 79.00

78.50 78.00 77.5 0 0.15 p a r t s 6740

0.45 p a r t s 6 7 %

Figure 11 Efect of A M P and standard dispersant on gloss for 70130 ClaylCaCO, coating

Table 2 Eflect of A M P on sheet gloss for 50150 claylcarbonate coating Component Clay CaCO, Dispersant AMP-95/Rheology Additive Latex Acrylic Thickener Optical Brightner Ca Stearate Melamine Formaldehyde YOsolids

PPh 50 50 0.15 0 13.5 0.3 1.5 0.8 1 66

PPh

50 50 0.15 0.2 13.5 0.3 1.5 0.8 1 66

Property

Stundard

With AMP-95

PH Brookfield @ 100rpm Hercules @ 4400 rpm Brightness (YO) Gloss Whiteness L* b K&N(%)

8.90 1612 53.9 89.7 55.6 87.6 94.8 -0.17 9

9.00 1490 54 90.5 58.6 88.1 94.6 -0.37 9

53

Thomas Gole

In a typical paper coating formulation with 67% solids containing a 70/30 ClaylCaCO, combination AMP-95 showed improved rheology performance especially at higher shear rates as indicated by Hercules viscosity (Figure 10)and also improved the gloss at 75" (Figure 11). Even in a 50/50 clay/CaCO, combination (with the CaCO, having a lower dispersant demand for alkaline dispersant) the gloss could be improved through the additional use of AMP-95 indicating an excellent dispersion and film forming process (Table 2). Further rheological data for a freesheet topcoating colour at 61.7% solids are shown in Figures 12 and 13.

6 Example of the Use of AMP-95TMin a Paper Coating Application - Post Added Figure 14 shows the effect when AMP-95TM was post added to an already formed coating colour in response to an in-mill problem. 4000

+

I

-

3500

-

3000

-

2500

-

2000

-!

a

E m

5

0,oo

0,lO

0,20

0,30

0,40

dosage (PPh)

Figure 12 Dispersant demand at low shear rate (Brookfield 100 rpm) 12

7 J 0,oo

0,lO

0,20

0,30

0,40

dosage (PPh)

Figure 13 Dispersant demand at high shear rate (Hercules, hob E , 4400 rpm)

A M P - 9 Y Mfor Mineral Dispersion

54

300

I

0,oo

0,20

0,lO

0,30

dosage (PPh)

Figure 14 A M P added to actual mill coating to solve problem

7 Improved Rheology AMP-95TMin paper coatings not only improves the rheology and gloss; it also has some side effects, which further enhance the coating performance. Due to its boiling point being higher than water, AMP-95TMensures stable pH control, alkalinity and controlled solvent evaporation during the drying process.

8 Summary AMP-95TMis liquid and can easily be dosed and incorporated into any coating formulation. The use level is typically between 0.15% and 0.25% based on solid content of the formulation. It is recommend using at least a part of AMP already during the grinding step to take full advantage of all its functionality. One typically will notice an improved wetting and easier filler incorporation with lower viscosities. In fact, because of the high performance, the formulator has to watch the viscosity carefully and has to maintain a viscosity level high enough to ensure sufficient energy input (shear force) for grinding. Adjusting the water content for the grind easily does this. In lots of cases AMP-95TMwas used as a post add and also improved performance was found. Through the use of the additive AMP-95TMseveral physical properties of the paper coating can be influenced and improved. Not only an ideal dispersion, viscosity and rheology profile can be achieved, due to the use of AMP-95TMalso improved runnability, less dispersion defects, improved coverage and gloss was found. Hand in hand with this goes significantly reduced defects as streaks, scratches and mottling. In a lot of cases the use of AMP-95TM enables the formulator to achieve higher pigment loading or a safer production run due to widening the application window with the same O/O solids.

Pigment Dispersion Technology for the Paper Industry D.J. Mogridge,l J.S. Phipps,] K.R. Roganl and D.R. Skuse2 l1MERYS MINERALS LTD, JOHN KEAY HOUSE, ST. AUSTELL, CORNWALL, PL25 4DJ, UK *IMERYS MINERALS INC., P O BOX 471,618 KAOLIN ROAD, SANDERSVILLE, GA, 31082, USA

1 Introduction Concentrated aqueous dispersions of fine, particulate kaolin and calcium carbonate are used extensively in the coating of paper. Currently more than 10 million tonnes of such dispersions are used annually in North America and Europe alone, and worldwide consumption is growing at around 3% per year. Generally, paper is coated to enhance its visual appearance. Firstly, coatings improve the optical properties of the blank sheet, i.e. its brightness (or whiteness), opacity and gloss. Secondly, they improve the physical properties of the sheet that will influence the quality of the final printed image, such as surface smoothness, porosity and ink absorption. In order to achieve these aims economically, paper coatings must be applied at high speeds and with minimal drying costs. Kaolin and calcium carbonate pigments are the major components of most paper coating formulations. Particle size distributions are chosen to maximise the optical benefits that they confer and to allow the production of formulations that can be easily pumped, handled and coated, Coating pigments are generally produced and transported as slurries, and so it is important to obtain dispersions with the highest possible solids content, both to minimise drying costs and the quantity of water transported. This paper examines the dispersant technology that is currently used in the industry and the mechanisms by which it functions, and illustrates ways in which this technology has developed and could be further improved in the future.

2 Paper Coating Technology Today Figure 1 is a schematic diagram of a typical blade coater. This is still the most common form of coating device used in the paper industry, although new 55

56

Pigment Dispersion Technologyfor the Paper Industry

Figure 1 Schematic diagram of blade coater

technologies such as jet coaters and metered roll applicators are becoming more popular and may in time supersede it. A continuous web of paper passes over a large rotating drum, and is brought into close contact with a coating ‘head’. The coating formulation, known in the industry as a coating ‘colour’, is circulated around the head and makes contact with the moving web of paper over a small region a few centimetres in length. At the end of this region a doctor blade is pressed against the paper, so that the desired coating thickness can be controlled by the pressure applied to the blade. A typical doctor blade is around 0.4mm thick, and the gap between it and the moving paper web is of the order of 10-20 pm. At coating speeds of up to 2000 m min- l, this translates to a shear rate in the order of lo6s-l over a period of 10 ps. Laboratory measurements of shear rates of this magnitude are not easily accessible, but the behaviour of a formulation at a shear rate of the order of lo4s-l will usually indicate whether or not a coating colour will run successfully. The optimum particle size for light scattering is of the order of the wavelength of light used and so most coating pigments have a mean particle size in the 0.3-0.8 pm range. They follow approximately log/normal size distributions, with a standard deviation of 0.3-0.6, which means that a typical coating pigment will have 95% by mass of its particles less than 2pm in diameter, 50% less than 0.5 pm and 25% less than 0.25 pm. Some 80-95%0 of the solids component of a coating colour will be pigment, with 5-15% latex or starch binder and minor quantities of other components such as low shear-rate thickeners, lubricants and fluorescent whitening agents. The papermaker will want to run the coating colour at the highest possible solids, which can be as high as 65% by mass where the pigment is primarily calcium carbonate. Coating colours are highly shearthinning materials, with apparent viscosities in the region of 1000mPas at 10 sand 50mPas at 104s-l. The normal dispersants used for both kaolin and calcium carbonate pigments are aqueous solutions of sodium polyacrylate. These are prepared by free radical polymerisation using various combinations of initiators and terminators which may be proprietary to the manufacturer. Number average molecular weights are

D.J. Mogridge et al.

57

generally less than 10000. The dispersant is added at an addition rate of up to 1% by mass of the pigment in order to obtain a Brookfield 100rpm apparent viscosity (which corresponds to a maximum shear rate of approximately 20 - I ) ' of 500mPas or less. Figure 2 and Figure 3 show some typical viscosity us. dose curves for calcium carbonate and kaolin slurries respectively. The values To, TI and T14refer to the viscosities as measured immediately, one hour and twentyfour hours after slurry preparation. Note that the optimum dose for kaolin is 1200

r-

1000 E

: h 8 .->

800

-

600

-

Y

u) u)

3

5

400 -

0

2 200 -

0.5

0.55

0.6

0.7

0.65

0.75

0.8

0.85

Dose I YO

Figure 2 Typical viscosity vs. dispersant dosefor N u polyacrylate on coating grade calcium carbonate (0.5 pm - 76% solids) 12000

10000 --

E

B

8

.-

8000 --

h

Y

u)

8

6000

--

4000

--

2000

--

5

rr 2 0

h

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Dose rate 'YO

Figure 3 Typical uiscosity vs. dispersant dose for N u polyacrylate on coating grade kaolin (0.6,urn -- 68% solids)

58

Pigment Dispersion Technology for the Paper Industry

substantially lower than that for calcium carbonate. This is believed to be because the dispersant only adsorbs on the positively-charged edges of the plate-like kaolin particles; the faces are already negatively charged over a wide pH range.

3 Colloid Chemistry of Sodium Polyacrylate Dispersed Calcium Carbonate Slurries In this section we give a description of the colloidal interactions which govern the state of dispersion and rheology of calcium carbonate slurries. The description is based on a series of fundamental experimental studies that have been described in more detail in previous publication^.^.^

3.1 Experimental Details A series of slurries of high solids, dispersed ground calcium carbonate were prepared. Firstly, a sample of Carrara marble was ground without dispersant to a mean size of 0.5,um. This was then filtered to a solids level of 76% (volume fraction 46%), before being thoroughly mixed with the required dose of sodium polyacrylate dispersant and adjusted to a constant solids level of 70%. Doses ranged from zero to 25 mg g-’ (2.5%) of calcium carbonate. Samples were then left for two days to reach a steady-state adsorption level. After this they were sheared and their viscosity (Brookfield RV, 100 rpm, spindle 3 ) was measured. The amount of polymer adsorbed on each sample was measured by pressure filtration through a 0.1 ,um filter, followed by analysis of the filtrate for residual polymer by gel permeation chromatography with refractive index determination. Particle zeta potentials were measured by taking a small sample of the solids from the centrifuge and re-suspending them in the supernatant prior to analysis in a Malvern Instruments ‘Zetasizer’. The concentration of all other types of ions in the supernatant was analysed by ICP atomic emission spectrosCOPY.

3.2 Results Figure 4 shows the viscosity of the samples as a function of dispersant dose. Note that the viscosity decreases more or less monotonically with dose throughout and beyond the normal commercial dosage range, but that the reduction in viscosity at doses above 1% is relatively small. Figure 5 shows the adsorption isotherm as calculated from the dose levels and the measurements of residual polymer in solution. The isotherm is of the high affinity type and can be divided into three regions. In region 1, at low polymer doses, almost all of the polymer added is adsorbed onto the particle surfaces and there is no detectable polymer in solution. In region 2, at intermediate doses, there is a small rise in the amount of polymer adsorbed but the majority of the extra polymer added in this region stays in solution. In region 3, at high polymer

D.J.Mogridge et al.

n

3-6

59

*

zi 0

3.4

2 h

Y

3-2

.3

w2

0

z >

.3

3.0

*

ba

5

2-4

2.2 0

5

1 0

1 5

20

25

Na Polyacrylate Dose / g kg-1 CaCO, Figure 4 Log (viscosity)vs. Dose for N u polyucrylute on 0.5 y m calcium carbonate at 70% solids

1600 PI

1400 w

,3

1200

6)

Y

3 1000

-5

800

0

za

$ -z d

600 400

200 0

Residual polyacrylate concentration / mg cm-3

Figure 5 Adsorption isotherm of N u polyacrylate on 0.5pm calcium carbonate at 70% solids

doses, the adsorbed amount rises again more steeply, whilst the solution concentration continues to rise substantially. Figure 6 shows the total ionic strength, calculated from the analysis of all ions in solution including the polyacrylate, as a function of polymer dose. Apart from a small deviation in region 1, the ionic strength rises almost linearly with polymer dose over the whole range. Ionic strength rises less rapidly in region 1 because the added polymer all adsorbs on the surface, and so the increase in ionic strength is caused only by release of sodium and carbonate/bicarbonate ions into solution.

60

Pigment Dispersion Technologyfor the Paper Industry

500

c?

E a

400

R

E 5ba

300

\

El

200

h

3; 1 0 0

0

5

10

15

20

25

Na Polyacrylate Dose / g kg-I CaCO, Figure 6 Effect of N u polyucrylate dose on solution ionic strength

-50

-40

-30

-20

0

5

1 0

1 5

20

25

Na Polyacrylate Dose / g kg-' CaCO,

Figure 7 Zeta potentiul of culcium curbonale us u function o f N a polyucrylate dose

Even at a dose level of 5 mg g-I some 80% of the contribution to ionic strength comes from unadsorbed sodium polyacrylate. Figure 7 shows the effect of polymer dose on the measured zeta potential. Zeta potential continues to rise quite steeply through regions 1 and 2, but then decreases rapidly in region 3. This sharp decrease could be a result of the reduction in ionisation of the polymer as the adsorption density rises; however, recently published studies4 on tethered polyacrylate chains at various surface

D.J. Mogridge et al.

61

densities suggest that at the pH of these experiments (8.5-9.5) the polymer should remain fully ionised despite the proximity of the charges to each other. The decrease in zeta potential is thus more likely to be caused by a shift in the position of the slip plane as the particles move or by specific ion binding to the adsorbed layer.

4 Discussion Zeta potential and ionic strength data were used to calculate the interparticle repulsion according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. According to these calculations, a maximum in the interparticle repulsion energy of around 120 kT should occur at the transition between regions 1 and 2, and thereafter the repulsion should fall rapidly, reaching zero at the end of region 2 and becoming attractive throughout region 3. Clearly this is not consistent with the viscosity data which suggest that the interparticle repulsion continues to rise through both regions 2 and 3. This indicates that the repulsion must also have a steric component which becomes increasingly important as the polymer dose is raised. In order to be able to include a steric contribution in the interparticle energy calculation, an estimate of the adsorbed layer thickness is required. This is very difficult to access experimentally; probably the only technique which might be able to provide an estimate is small-angle neutron scattering which was beyond the scope of this work. As a result, a theoretical estimation of the thickness was made, based on a few key observations. This is described below. Figure 8 shows the radius of gyration (R,) of sodium polyacrylate in solution as a function of ionic strength, calculated from intrinsic viscosity measurement^.^ It can be seen that R, varies almost linearly with the inverse square root of the ionic strength ( I ) . By fitting the data to a simple second-order polynomial, the value of R, in solution was interpolated across the whole of the ionic strength range studied. Likewise, the area per molecule or molecular ‘footprint’ of the adsorbed polymer in regions 1 and 2 was calculated from the adsorbed amount and surface area of the sample and this was then converted to the apparent R, of the adsorbed polymer by multiplying by a simple geometric conversion factor. This is also shown in Figure 8. As with the solution polymer, the of R, the adsorbed polymer in a completed monolayer (region 2) varies almost linearly with I-1/2. The R, data for the adsorbed polymer in the completed monolayer was fitted to a polynomial of equal gradient to that fitted to the data for the solution polymer, and this was then extrapolated into region 3 in order to obtain a value for the R of the polymer in the region of multilayer adsorption. It was assumed that in tiis region all polymer molecules in completed monolayers have the same R, regardless of which layer they are in. The R, of the adsorbed polymer in a completed monolayer is lower than the corresponding R, in solution because the polymer chains are laterally compressed by the presence of neighbouring polymer molecules. The assumption was also made that the polymer occupies the same volume on the surface as in the bulk at each value of ionic strength, so

62

Pigment Dispersion Technology,fiwthe Paper Industry

4 -0

3-5

3.0 0 Q

2.5

2 2

6

4

I-”2 Figure 8 Radius of gyration of N u polyacrylate in solution and adsorbed on calcium carbonate surface as a function of ionic strength

that laterally-compressed adsorbed polymer chains occupy a cylindrical volume, the length of which (and thus the thickness of the adsorbed monolayer) can be calculated from the polymer volume and the value of R,. The thickness of the adsorbed polymer layer at each dose level was thus estimated from the following sequence of steps: 1. The number of polymer molecules adsorbed per unit area was calculated from adsorption isotherm data and the surface area per unit mass of the sample. 2. The radius of gyration and area per molecule in a completed monolayer was determined from the polynomial fit of adsorbed R, us. I . 3. The number of molecules in a completed monolayer was then calculated and hence also the number of completed layers and the number of molecules in the outer, incomplete layer. 4. The area per molecule in the outer, incomplete layer was calculated. 5. By equating the polymer volume in solution with the polymer volume on the surface, the thickness of each layer was calculated and these values were summed to obtain the total layer thickness.

This is shown schematically in Figure 9. The total layer thickness calculated using this approach is shown in Figure 10. The most notable feature is the substantial contraction of the layer thickness in region 2. This occurs because in this region the adsorbed amount remains almost constant, whilst the solution ionic strength rises rapidly. The contraction of the layer in this region implies that both the steric and electrostatic components of the interparticle repulsion are reduced by increasing the solution ionic strength. In region 3, where multilayers are forming, the effect of increasing ionic strength

D.J. Mogridge et al.

63

R (sol) .cc

Polymer in solution

-

R (ads)

Polymers in layers

Surface Figure 9 Schematic representation ofmodel used for estimation ofpolymer layer thickness

is offset by the increasing amount of polymer adsorbing onto the surface, and hence the layer thickness increases. Using this data for the layer thickness, the total energy of interaction was calculated by summing the electrostatic and steric contributions, the latter of which was calculated according to the method of OttewilL6 Total interaction energies at three values of interparticle separation are shown in Figure 11 as a function of polymer dose. From Figure 11 it can be seen that the inclusion of a steric term changes the calculated interparticle repulsion substantially. Whereas the electrostatic-only

Na Polyacrylate Dose / g kg-1 CaCO, Figure 10 Calculated lajer thickness as afunction ofpolymer addition lecel

Pigment Dispersion Technologyfor the Paper Industry

64 400 -at

350

+at

6nm 12 nm

/

300

. 5

h

P

250

r

;200 o_

2 P Iy)

150 100

50

0

1

0

5

10

15

20

25

Na Polyacrylate dose/g kg-'

Figure 11 Calculated total interaction energies at ,fixed interparticle separation as a function of polymer dose

calculation predicted that the maximum repulsion would have shrunk to zero at a dose level of lOmgg-', the combined calculation predicts that close to the surface it remains above 100 kT at all doses, and rises substantially at the onset of multilayer formation. Hence by including a steric term the calculation agrees with the observation that the slurry remains dispersed at all doses above 1 mgg-l. However, the model also predicts that the repulsion should reach a minimum at a dose of around 8 mg g-l, and this is clearly not indicated by the viscosity data either for this sample or for paper-coating grade calcium carbonates in general. There are a number of assumptions made in the model that are questionable, and these are probably responsible for the discrepancy between the predicted and observed behaviour of the slurries. For example, the steric contribution has been calculated assuming that the adsorbed layer has a well-defined thickness. For adsorbed polymers this is unlikely to be the case, as the volume fraction profile of the polymer will decay gradually as a function of distance from the surface. Furthermore, it was assumed that the effective ionic strength in the adsorbed polymer layer is the same as in solution. However, this also is unlikely since one of the main components of the solution ionic strength is the polymer itself, and unadsorbed polymer will be excluded from the adsorbed layer. Finally, the connectivity of the charged groups on the polymer was not considered, so its contribution to ionic strength may have been overestimated. Despite these misgivings, the model is useful in that it highlights a number of important points. Firstly, it confirms that steric repulsion is the main driving force for dispersion at the higher polymer doses used, and thus explains the continued performance of the polymer as a dispersant even at relatively high values of ionic strength. Secondly, it emphasises that increasing ionic strength is

D.J. Mogridge et al.

65

harmful not just to the electrostatic component of the repulsion, but also to the steric component because it compresses the adsorbed layer. As a result, it shows that to improve the effectiveness of the dispersant measures should be taken which maximise the amount of polymer on the particle surface and the layer thickness, and which minimise the solution ionic strength.

5 Application of Model Conclusions to Slurry Dispersant Optimisation From the preceding discussion, the most obvious way to improve the performance of polyacrylate-dispersed calcium carbonate slurries is to reduce the solution ionic strength by reducing the amount of non-adsorbed polymer in solution. An efficient way of increasing the amount of polymer adsorbed whilst minimising the amount in solution and hence minimising ionic strength is to increase the affinity of the polymer for the particle surface. For polyacrylate on calcium carbonate, this can be done quite simply by changing the degree of neutralisation of the added p ~ l y r n e ras , ~ any acid groups will react directly with the particle surface and thus bind strongly to it: 2R-CO,H

+ CaCO, -+ (R-CO,),Ca + CO,

Furthermore, binding of the polyacrylate in this way does not lead to the release of sodium ions into solution as it does when the 100% sodium neutralised polymer is used. The final equilibrium slurry pH remains around 8.5 because it is buffered by the carbonate. Although this approach does lead to stronger binding of the polymer, it will also reduce the amount of free charges on the adsorbed chains and most likely lead to a flatter conformation. As can be seen from Figure 12, as a result of these two conflicting effects there is an optimum range of initial neutralisation that leads to low slurry viscosities. A similar situation is seen for kaolin, although in this case it is the final slurry pH which dictates the dispersant performance rather than the initial state of the polymer, since the particles do not buffer the solution in the same way as calcium carbonate. As pH is decreased the edges of the kaolin particles become more positvely charged which leads to stronger adsorption of the negatively charged polymer. However, if the pH gets too low the degree of ionisation of the polymer decreases which weakens the adsorption. Likewise at high pH the polymer is highly negatively charged, but as the positive charge on the particles also reduces again the affinity of the surface for the polymer is reduced. As can be seen in Figure 13, there is an optimum pH range of 6-8, outside of which the viscosity, particularly after standing, begins to rise. One final way in which to reduce the amount of polymer in solution is to use a narrow molecular weight distribution. It is well known that narrow distributions give sharper adsorption isotherms. Commercial polymers are made by free

66

Pigment Dispersion Technologyfor the Paper Industry

Figure 12 EfSect of degree of neutralisation of polyacrylic acid on viscosity of dispersed culcium carbonate (0.5 pm, 76% solids)

4

4.5

5

5.5

6

6.5

7

7.5

a

8.5

9

PH

Figure 13 EfSect of p H on viscosity qf polyacrylate dispersed kaolin (0.6 pm, 70% solids)

radical polymerisation methods that do not naturally lead to narrow distributions, but sample polydispersity can be reduced by molecular weight fractionation techniques. Figure 14 shows the effect of such a fractionation on the viscosity of a calcium carbonate slurry.

6 Summary The paper industry requires high solids dispersions of kaolin and calcium

D.J.Moyridye et al.

67

4000 3500

3000

e 2500 z1 w

ul

2

2000

5

$

1500

c

;1000 0

500

0 TO

T1

T24

L

Figure 14 Efect of polymer M wfractionation on viscosity of dispersed calcium carbonate (0.5 pm, 76% solids)

carbonate with good rheology for both cost and quality reasons. Sodium polyacrylate is the most widely used dispersant for this purpose because it is both cheap and effective. Fundamental studies of the effects of sodium polyacrylate have shown it to be an ‘electrosteric’ dispersant, for which the steric component of interparticle repulsion is the dominant one at the typical dose rates used commercially. In such dispersions the main contributor to solution ionic strength is unadsorbed polyacrylate. As well as representing a waste of valuable dispersant polymer, this unadsorbed material also reduces the effectiveness of the adsorbed material as a steric dispersant by compressing the conformation of the adsorbed layer. Measures taken to minimise the amount of unadsorbed polymer in solution have been shown to improve the viscosity performance of both kaolin and calcium carbonate slurries. These include adjustment of the pH or initial neutralisation of the polymer to increase its affinity to the surface and fractionation of the polymer to reduce the width of its molecular weight distribution.

References 1. H.A. Barnes, Applied Rheology, 2001,11(2),89. 2. K.R. Rogan, A.C. Bentham, G.W.A. Beard, I.A. George and D.R. Skuse, Progress in Colloid and Polymer Science, 1994,97,97. 3. K.R. Rogan, A.C. Bentham, T.A. George and D.R. Skuse, Colloid and Polymer Science, 1994,272,1175. 4. E.P.K. Currie, N.J. Avena and M.A. Cohen Stuart, in ‘Polyelectrolytes: Proceedings of the 50th Yamada Coyference’, I. Nada and E. Kokufuta (eds.), Yamada Science Foundation, 1999,384.

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Pigment Dispersion Technologyfor the Paper Industry

5. K.R. Rogan, Colloid and Polymer Science, 1995,273, 364. 6. R.H. Ottewill, in ‘Scientijic Methods for the Study of Polymer Colloids and Their Applications’, F. Candau and R.H. Ottewill (eds.), NATO AS1 Series, Kluwer Academic, Dordrecht, 1990,129. 7. 0.Gonnet, G. Ravet and J. Rousset, U S PAT., 4840985.

Selective Processing

An AFM and XPS Investigation of the Selective Flocculation of Kaolinite from a Mineral Mixture J.S. Dalton,' G.C. Allen,l K.R. Hallam,l N.J. Elton,2 J.J. Hooper3 and D.R. Skuse4 IINTERFACE ANALYSIS CENTRE, UNIVERSITY OF BRISTOL, 121 ST. MICHAELS HILL, BRISTOL, BS2 8BS, UK 2SCHOOL O F ENGINEERING & COMPUTER SCIENCE, UNIVERSITY OF EXETER, NORTH PARK ROAD, EXETER, DEVON, EX4 4QF, UK 3TMERYS MINERALS LTD, PAR MOOR LABS, PAR MOOR ROAD, ST. AUSTELL, CORNWALL, PL24 2SQ, UK 41MERYS MINERALS INC., PO BOX 471,618 KAOLIN ROAD, SANDERSVILLE, GA, 31082, USA

1 Introduction Mineral segregation in industry relies heavily on the selective adsorption of macromolecules onto the surfaces of those minerals that have particular industrial applications. This selectivity is governed mainly by the surface chemistry of the mineral and the type of polymer used as a flocculant.132 Effectiveness of flocculation depends upon the charge, concentration and molecular weight of the polymer, and also the pH and salt concentration of the clay suspension. The bonding between the anionic flocculant polyacrylamide (PAM) and clay mineral surfaces has been effectively reviewed recently by Hocking et al.3 and the reader is referred to this should they require an in-depth literature review. For more information on general colloidal chemistry of clay suspensions the reader is referred to the review of Luckham and R o ~ s i . ~ There is general agreement that the main mechanism for the adsorption of cationic polymers onto clay surfaces occurs by charge ne~tralisation.~ There is less agreement on the adsorption of anionic polymers, such as hydrolysed polyacrylamide (HPAM), onto mineral surfaces. It has been suggested that this occurs by cation bridging whereby divalent ions bridge negatively charged sites on the clay and carboxyl groups on the p ~ l y m e r .However, ~.~ work by Peng and Di has shown that Ca2+ and A13+ actually inhibit kaolin flocculation as these 71

72

An A F M and X P S Investigation of the Selective Flocculation of Kaolinite

ions decrease the charge density and extendibility of the p01ymer.~Work of Pefferforn and Nazbar showed that adsorption is governed by hydrogen bonding between the amide group on HPAM and hydroxyl groups at the surface of the clay.8p1 This, in turn, depends on the pH of the mineral suspension. A recent paper by Laird12 investigated the efficacy of HPAM flocculation of kaolinite, illite and quartz by carrying out visible absorption experiments. He concluded that HPAM more effectively flocculates kaolinite than quartz or illite. This was also the conclusion of previous work by Allen et al. who studied the adsorption of HPAM onto kaolinite, quartz and feldspar at various HPAM concentrations and solution pH by X-ray photoelectron spectroscopy (XPS).'3p1 Much of the previous work on polyacrylamide adsorption onto aluminosilicates monitored the adsorbed amount by viscometry,' carbon analysis' 7 > 1 and radiotracer techniques." These methods rely on following adsorption by subtraction from that detected in solution. This paper contributes to the literature by quantifying anionic polymer adsorption onto the clay minerals kaolinite, feldspar, mica and quartz by X-ray photoelectron spectroscopy (XPS). XPS measures the sorbed amount directly rather than by a subtraction technique. This enables an insight into how effective selective flocculation is for obtaining kaolinite from a mineral mixture. Atomic force microscopy (AFM) is also used to image polymer adsorption onto mineral surfaces and the effectiveness of this technique applied to mineral surfaces is discussed here.

2 Materials and Methods 2.1 Materials 2.1.1 Minerals. The polymer used in this study is partially hydrolysed polyacrylamide (HPAM), supplied by Allied Colloids Ltd, Low Moor, Bradford, West Yorkshire, England, UK. Approximately 10 mol% of the monomer units are hydrolysed and are present as sodium acrylate with a polyacrylamide-coacrylate copolymer having a molecular mass of approximately 6 x 106D.The study minerals quartz, kaolinite and feldspar were collected from the Lee Moor China Clay Pit in Devon, UK. Mica was obtained in sheet form from Agar Scientific. Minerals were ground such that a size distribution ranging from sub-micron to millimetre particles were obtained. For the experiments described here, it is desirable to use monodisperse minerals. To this end, a sedimentation technique was used to obtain minerals in the particle size range (effective Stokes radius) (i) 10-20 pm; and (ii) above 20 pm.

2.1.2 Mineral Flocculation for X P S Analysis. A stock solution of 500 ppm HPAM in water was prepared and left to diffuse and condition for 1 hour due to the viscosity of the solution. 1g of the mineral was made into a slurry with 10 cm3 distilled water and this was then made up to 50 cm3 in a measuring cylinder. The solution pH was then altered to 9.0 with the relevant amount of 0.1 M NaOH

J . S . Dalton et al.

73

solution. An amount of 500ppm HPAM was added to obtain the desired concentration and the cylinder is made up to 100cm3. The cylinder was shaken and rotated for 1 minute, and left to stand for a further 30 minutes by which time the mineral had settled by gravity. The supernatant HPAM solution was then decanted. The resulting mineral slurry was transferred to a conical flask and washed three times with distilled water, each time the supernatent being decanted off. This rinsing process ensured any unadsorbed or very weakly adsorbed HPAM was removed. The remaining slurry was deposited on a silica support and left to dry for 24 hours, ready for XPS investigation. For imaging XPS, this process was carried out on a kaolinite and coarse quartz (above 20 pm stokes radius) 1: 1 mix. 2.1.3 Mineral Preparation for A F M Imaging. Both untreated and polymer treated mineral samples were attached to metallic stubs using a rapid acting araldite from Ciba Speciality Chemicals. The araldite permitted careful placing of the minerals such that macroscopically flat areas were accessible for AFM analysis.

2.2 Methods 2.2.1 X-ray Photoelectron Spectroscopy ( X P S ) . XPS analyses the near-surface of materials with analysis depths up to 5nm. It can provide quantitative chemical information as well as oxidation and structural environments on all elements apart from hydrogen and helium.' XPS spectra were acquired using a VG Scientific Escascope spectrometer using Mg K , X-rays (300W; 15kV, 20 mA) and a hemispherical electron energy analyser. Data acquisition and manipulation were carried out using manufacturer-supplied VGS5250 software. During acquisition of spectra, the pressure in the main chamber was maintained at 8 x 10-9Torr to ensure a clean sample surface. Charge referencing was carried out against adventitious hydrocarbon (C 1s = 284.8 eV)19from pump oil contamination. Imaging XPS was carried out in constant retard ratio mode with a 1200pm field-of-viewgiving < 50 pm resolution. A 500 eV flood gun was used to locate the region of interest. Spectra were then acquired in this region to obtain specific photoelectron peaks and backgrounds. The background is a few electron-volts lower in energy than the photoelectron peak where the count intensity is purely due to the averaged effect of inelastically scattered electrons. A resultant image of the background subtracted from peak map can help in reducing the effects of surface topography. 2.2.2 Atomic Force Microscopy ( A F M ) . The AFM20321images were obtained on a Digital Instruments Dimension 3100 microscope which was mounted on an air cushioned metal table to reduce external vibrations. Images were collected and processed using Nanoscope version 4.421-1 software. The instrument was operated in tapping mode (TMAFM)22.23in air using etched silicon cantilevers with a spring constant of 50Nm-l. Tapping mode was used to minimise the lateral forces the tip exerted on the sample surface which could disrupt and

14

A n AFM and X P S Investigation of the Selective Flocculation of Kaolinite

deform the polymer layer. A TMAFM scan allowed the acquisition of a height image and the corresponding phase image simultaneously. The phase contrast gave information on the viscoelasticity (material contrast) helping to distinguish between features of the hard substrate surface and the adsorbed soft polymer.24 A high setpoint, ensuring intermittent contact, and a slow scan rate (0.50Hz) were chosen. 2.2.3 Scanning Electron Microscopy ( S E M ) . The SEM used was a commercial Hitachi S2300 instrument with a tungsten hairpin filament. An accelerating voltage of 25 keV was used and samples were gold coated to eliminate charging.

3 Results and Discussion 3.1 Monodispersed Minerals SEM images of the monodispersed minerals are shown in Figure 1. Unfortunately it was not possible to obtain kaolinite particles in the desired size range (10-20 pm) due to flocculation occuring in the sedimentation process. It was not desirable to chemically treat the surface of the kaolinite to prevent this, as this would affect later results. To this end, the kaolinite was used untreated, as received, with the smaller size fraction shown below. Monodispersed minerals will allow direct comparison in HPAM adsorption isotherms.

Figure 1 (a) Mica (50y m marker);(b)feldspar(100 ym marker); (c) quartz (50 y m marker); all monodisperse with effective Stokes radius 10-20 ym. ( d ) Kuolinite (5ym marker)

J.S. Dalton et al.

75

3.2 XPS: HPAM Adsorption Isotherms 3.2.1 Minerals. X-ray photoelectron spectra were run on the untreated minerals to obtain peak binding energies and atomic percent in the surface. This is presented in Table 1. It should be noted that the surface carbon (C I s ) is due to adventitious hydrocarbon impurity from the UHV diffusion pump. The nitrogen impurity present on the surface of the minerals was organic, probably amine or amide, and is thought to be associated with the origin of the minerals. Many organic molecules, for example pesticides, are seen to sequester into weatheringproduced ‘dead-end’ pores at mineral surfaces.24The XPS spectrum of a HPAM film was consistent with its chemical structure. 3.2.2 Minerals Treated with Polyacrylamide. Minerals were treated with HPAM as outlined in Section 2. Any unadsorbed or weakly bound HPAM was removed from the mineral by washing and rinsing, Adsorption on the mineral surface was monitored by the N I s photoelectron peak from the HPAM. To compare HPAM adsorption between minerals, peak integration was used to obtain surface atomic YOof N I s , Si 2 p and A1 2p. The ratio Nls: (Si2p + A12p) was used to normalise all the samples, which eliminates the contribution of C I s hydrocarbon contamination from the analysis. Multiple XPS scans gave error values as rfI 5% on atomic surface percent and & 0.2 eV on binding energy. Figure 2(a) shows how the adsorbed amount of HPAM on kaolinite increased with bulk concentration HPAM. For clarity, the 200 ppm measurement was omitted, but the peak intensity was not seen to rise above 50ppm. It would appear that there is surface saturation above this 50 ppm threshold limit. Figure 2(b) shows an HPAM adsorption isotherm for both kaolinite and feldspar. Neither the quartz or mica showed any adsorbed HPAM up to a bulk concentration of 200ppm. The N Is atomic percent did not vary from the background adsorption at 0 ppm, at least within the error margin in these experiments.

Table 1 Peak binding energies and surface atomic percent for the minerals NIs

c Is

532.2 47.1

399.8 0.4

294.8 36.6

74.4 1.5

532.4 49.5

399.8 0.7

284.8 27.7

102.5 13.6

74.2 9.7

531.8 57.3

292.9 4.9

399.8 0.3

284.8 14.3

102.5 15.7

74.2 3.5

53 1.8 49.8

292.9 4.4

399.8 0.2

284.8 26.4

Si2p

A12p

01s

kaolinite binding energy/eV atomic %

102.8 9.2

74.4 6.7

quartz binding energy/eV atomic YO

102.8 20.5

mica binding energy/eV atomic % feldspar binding energyjeV atomic O/O

K2p

76

A n A F M and X P S Investigation ofthe Selective Flocculation of Kaolinite (4

(b)

4400

4200 v)

c

4000 3800 U

3600

.$ -

z 3200 406

2

m E l

3400

404

402

400

398

binding energy I eV

396

394

OC 0

7

20 40 60 80 100 bulk HPAM concentration / p p m

120

Figure 2 ( a ) Using the N 1s photoelectron peak to follow the adsorption of HPAM on kaolinite. Inset shows hulk HPAM concentration in order of peak intensity. (h) HPAM adsorption isotherm onto kaolinite (squares)and feldspar (circles)

3.3 XPS Imaging: Selective Adsorption of HPAM in Mineral Mixtures In order to monitor the selective adsorption of HPAM onto one mineral in preference to another, imaging XPS was carried out on a mixture of kaolinite and quartz. Figure 3 shows the backscattered electron image of the kaolinquartz matrix in addition to the Si 2 p and A1 2 p photoelectron images. The field of view is 1200pm. Both (b) and (c) are resultant photoelectron images, the background having been subtracted from the peak image. Using this technique it is possible to differentiate the larger quartz crystals embedded in the kaolin matrix due to the presence and absence of Si and A1 respectively. Note, however, there is a signal from A1 2 p on the surface of the quartz crystal.

Figure 3 (a) Backscattered electron image ($a kaolin-quartz mix; (b)Si 2p photoelectron peak image; (c)A1 2p photoelectron peak image

J.S. Dalton et al.

77

It was attempted to gain N Is images to identify where the HPAM adsorbed, but images were generally poor. Due to the topography of the sample there was a large amount of shadowing. Photoelectrons are not emitted from this region giving poor images. It appears that when the kaolinite flocculates, it entraps other minerals within the flocs, hence minerals such as quartz will have a covering of kaolinite on their surface. This is seen in the SEM images in Figure 4. Therefore the N Is map will represent the HPAM adsorbed on the kaolinite covering the quartz. XPS measurements on mineral mixtures of this kind proved an ineffective means of monitoring HPAM adsorption.

3.4 AFM Analysis of HPAM Adsorbed to Kaolinite A review on AFM imaging of mineral surfaces has been previously given by In order to effectively image minerals in the micron size range it is essential that the surface is relatively flat,26 at least less than 5pm roughness, otherwise image quality is poor. AFM is capable of imaging both large and small areas of mineral surfaces and this is demonstrated in Figure 5. Figure 6 shows a typical AFM topographical (a) and phase (b) image of untreated kaolinite. The phase image shows no contrast on the plates, which confirms a clean surface. The hexagonal nature of the plates is seen clearly in both images. The HPAM treated kaolinite is also shown in (c) and (d). Initially concentrating on the topological image (c) it can be seen that these plates are less distinguished than the untreated and this was due to adsorbed HPAM. The phase contrast (d) occurs mainly from topographical polymer layer effects indicating that the adsorption on the basal plane is at least of monolayer coverage over most of the plate. In addition the characteristic ‘booksheets’ on the lateral faces of kaolinite are also seen. The adsorption to the basal surfaces of the kaolinite is generally deemed to be due to hydrophobic bonding and also hydrogen bonding to the aluminol groups.

Figure 4 SEM images of a kaolinite-quartz-mica mineral mix aftsrjocculution by HPAM. The kaolin covers other minerals ‘encapsulating’them inside thepocs

An AFM and X P S Investigation of the Selective Flocculation of Kaolinite

78

- 10

n200*o

-1.00

0 0

1.00

rO

I

2.00

10

0

CLm

Clm

Figure 5 AFM imaging ofkaolinite

b) 500

500

0

250

0.0

, 2 5 . 0 des

"*

75.0

-250

,

"*

500 70

0

250

5000

"*

c>

75 0 n*

250

0

250

0.0

0 500 nn

1 7 . 5 deg

"* 250

0

0 . 0 deg

0 250

500

n*

Figure 6 Topographic and phase imaging of untreated and H P A M treated kaolinite

3.5 Adsorption Mechanisms It is seen here that HPAM strongly adsorbs to kaolinite, causing flocculation, and also has affinity to the feldspar surface. Little or no adsorption was observed on the quartz or mica surface. Any adsorption mechanism must account for this.

J . S . Dalton et al.

79

The schematic in Figure 7 shows the suggested HPAM bonding mechanism to the kaolinite surface at low and high pH at both the siloxane and aluminol sites. Previous IR measurements have shown that interactions between polymer amide groups and oxygens of siloxanes were found to be very weak.27In addition, at pH 9.0 there will be repulsion between the carboxylate groups on the HPAM and the exposed > Si-0- group. This explains little bonding to the quartz. Lee et al. measured the adsorption of polyacylamide onto gibbsite and observed a relatively high surface coverage.18It would appear that the strongest hydrogen bonding occurs at the aluminol sites on these minerals.

4 Conclusion XPS can be used to quantify HPAM adsorption onto minerals at various polymer bulk concentrations. It is seen here that kaolinite has twice the affinity for HPAM than feldspar at pH 9.0 and 50ppm. Little or no adsorption was monitored on the surface of quartz or mica. Imaging XPS to monitor selective adsorption of mineral mixes proved difficult. Flocculating a mineral mixture of kaolinite, mica and quartz caused the kaolin flocs to encapsulate the other minerals. This created a layer of kaolin on the quartz and mica prohibiting polymer mapping on their surfaces. It is shown that the effectiveness of the kaolin recovery is more strongly affected by encapsulation of other minerals during flocculation rather than the selective adsorption process. Providing a flat surface can be achieved, tapping mode AFM topological and phase imaging can be used to investigate HPAM adsorption and mineral surfaces. Information on the film height, the strength of interaction between polymer-mineral surface and an indication of the adsorption process may be obtained. HPAM adsorption on kaolinite was observed to form a flat uniform layer (a)

LowpH

(b) High pH

NH,-

e,' R

Figure 7 Bonding mechanisms.for HPAM onto kaolinite

80

An AFM and X P S Investigation of the Selective Flocculation o f K aolinite

on the basal surface and is thought to occur via hydrogen bonding between the amide C=O group and the gibbsite Al-OH layer.

Acknowledgements The authors would like to thank Mr Heiko Haschke and Mr Edward Leach for assistance with the AFM imaging.

References 1. G.J. Fleer, M.A. Cohen-Stuart, J.M.H.M. Scheutjens, T. Cosgrove and B. Vincent, Polymers at Interfaces, Chapman and Hall, London, 1993. 2. E. Dickinson and L. Eriksson, Adv. Coll. Int. Sci.,1991,34, 1. 3. M.B. Hocking, K.A. Klimchuk and S. Lowen, J.M.S.-Rev. Macromol. Chem. Phys., 1999, C39(2), 177. 4. P. Luckham and S. Rossi, Adv. Coll. Int. Sci., 1999,82,43. 5. S.M. Aly and J. Letey, Soil Sci. SOC.Am., 1998,52, 1453. 6. V.C. Farmer, Soil Sci., 1971, 112,62. 7. F.F. Peng and P. Di, J . Coll. Int. Sci., 1994,164,229. 8. L. Nabzar, E. Pefferkorn and R. Varoqui, Coll. Surfaces, 1988,30,345. 9. L. Nabzar, E. Pefferkorn and R. Varoqui, J . Coll. Znt. Sci., 1984,102,380. 10. E. Pefferkorn, L. Nabzar and A. Carroy, J . Coll. Znt. Sci., 1985,106,94. 11. L. Nabzar and E. Pefferkorn, J . Coll. Int. Sci., 1985,108,243. 12. D.A. Laird, SoilSci., 1997,162, 826. 13. G.C. Allen, K.R. Hallam, J.R. Eastman, G.J. Graveling, V.K. Ragnarsdottir and D.R. Skuse, Surf: Int. Anal., 1998,26, 518. 14. G.C. Allen, J.R. Eastman, K.R. Hallam, G.J. Graveling, V.K. Ragnarsdottir and D.R. Skuse, Clay Minerals, 1999,34, 51. 15. G.J. Graveling, K.V. Ragnarsdottir, G.C. Allen, J. Eastman, P.V. Brady, S.D. Balsey and D.R. Skuse, Geochim. et Cosmochimica Acta, 1997,61,3515. 16. G. Girod, J.M. Lamarche and A. Foissey, J . Coll. In. Sci., 1988, 12,265. 17. J. Lecourtier, L.T. Lee and G. Chauveteau, Coll. Surfaces, 1990,47,219. 18. L.T. Lee, R. Rahbari, J. Lecourtier and G. Chauveteau, J . Coll. Int. Sci., 1991, 147, 351. 19. D. Briggs and M.P. Seah, Practicul Surjiuce Analysis (2nd edn.), Vol. 1, John Wiley, Chichester, 1992. 20. G. Binnig, C.F. Quate and C. Gerber, Phys. Rev. Letters, 1986,56,930. 2 1. R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cam bridge University Press, Cambridge, 1994. 22. Q. Zhong, D. Inniss, K. Kjoller and V.B. Elings, Surf: Sci., 1993,290, L688. 23. D. Anselmetti, M. Dreier, R. Luethi, T. Richmond, E. Meyer, J. Frommer and H.-J. Guentherodt, J . Vac. Sci. Techno!., 1994, B12, 1500. 24. R.P. Schartzenbach, P.M. Geschwend and D.M. Imboden, Environmental Organic Chemistry, Wiley-Interscience, New York, 1993. 25. P.A. Maurice, Coll. Surfaces, 1996, 107, 57. 26. A.E. Blum, in Scanning Probe Microscopy qf Clay Minerals, Nagy and Blum (eds.), Clay Min. SOC.,London, 1994. 27. J.P. Burelbach, S.G. Bankoff and S.H. Davis, J . Fluid Mech., 1988, 195,463.

The Nature of Adsorption Sites on Unrefined and Ball Milled Kaolin. A Diffuse Reflectance Infrared Fourier Transform Spectroscopic Study C. Breen,l J. Illes,l J. Yarwoodl and D.R. Skuse2 'MATERIALS RESEARCH INSTITUTE, SHEFFIELD HALLAM UNIVERSITY, SHEFFIELD, S1 1WB, UK 21MERYS MINERALS INC., PO BOX 471,618 KAOLIN ROAD, SANDERSVILLE, GA 31082, USA

1 Introduction Mechanical milling of mineral samples is often utilised to modify surface and bulk properties such as particle size and surface area. However, other changes accompany the milling process such as structural alterations, complete or partial amorphisation of the material together with an increase in the surface energy and surface reactivity which may lead to enhanced chemical reactivity. The effect of grinding on kaolin has received much attention and the results reviewed in Dry grinding of kaolin is known to fracture the individual crystals leading to an increase in surface area. However, scanning electron microscopy suggests that this increase may be attentuated as the small particles aggregate even at short grinding times. Indeed, mechanochemical treatment for as little as 1 h can reduce the temperature at which the technologically important kaolin to mullite transformation O C C U ~ S . ~ A wide variety of instrumental techniques, including X-ray diffraction, thermal have analysis, electron microscopy, M AS-NMR and infrared been employed at different levels of complexity to investigate the effects of mechanochemical treatment on kaolin. Unfortunately, vibrational spectroscopy has only been used at a superficial level in the study of milled kaolin despite the considerable contribution that it has made to the understanding of the structure and reactivity of kaolin itself. Thermal analysis, and to a lesser extent infrared spectroscopy, has shown that grinding kaolin reduces the number of structural hydroxyls and increases the amount of sorbed water. However, the infrared studies have been restricted to 81

82

The Nature of Adsorption Sites on Unrefined and Ball Milled Kaolin

transmission studies using KBr discs at room temperature so little information concerning the nature and the thermal stability of the different types of sorbed water has emerged? In this study variable temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS), supported by X-ray diffraction (XRD), scanning electron microscopy (SEM) together with thermogravimetric analysis (TGA) have been used to investigate the nature and thermal stability of water molecules associated with kaolin which has been milled for 3, 10 and 30 minutes at room temperature. The 1500 to 1700cm-I region of the DRIFTS spectra of the milled kaolins has been particularly informative identifying at least two separate water populations of differing thermal stability which are reflected in the OH stretching region (3000-3600 cm-').

2 Materials and Methods The kaolin sample used was supplied by English China Clays International Ltd. (now IMERYS). X-ray diffraction data indicated the presence of a mica impurity ( < 5 wt%). Separate 2 g samples were milled in a Retsch MM2 ball mill grinder in sealed containers at 45Hz for 3, 10 and 30 minutes and transferred to airtight containers. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) spectra were recorded using a Graseby Selector DRIFTS accessory and its associated environmental chamber. Spectra of samples (5 wt% in dry KBr) were recorded at 25,50,75,100,125,150,200,250,300,350,400 and 450 "C. The sample chamber was purged with dry nitrogen whilst 276 scans were collected from 400 to 4000 cm- at 4 cm- resolution. Samples were immersed in reagent grade pyridine overnight and the excess pyridine allowed to evapourate before preparing samples and collecting spectra as above. Thermogravimetric traces (7 mg sample weight) were obtained using a Mettler TG50 thermobalance equipped with a TClOA processor. The fitting of the DRIFTS spectra was accomplished using the Grams software, Galactic Industries Ltd.

'

3 Results and Discussion 3.1 Kaolin Water Samples Powder X-ray diffraction traces (not illustrated) confirmed; (i) the presence of a crystalline mica impurity which degraded rapidly with milling time and (ii) that the kaolin became less crystalline as the milling time progressed but that the diagnostic 001 and 002 peaks were still present (at much reduced intensity) after 30 minutes milling. The thermogravimetric trace for unmilled kaolin is typical in that the layer dehydroxylation occurred as a single event (a loss of 14wt%) beginning just below 500°C (Figure 1). The total weight loss increased slightly as the milling time progressed but the weight loss associated with the unchanged kaolin decreased. Moreover, the amount and rate of water loss below 400 "C increased

83

C. Breen et al.

I

Figure 1 Thermogravimetric curves for the unrefined and milled kuolins

with milling time. This behaviour correlates well with the work of others.lP4 Figure 2a shows that the unrefined kaolin exhibited four characteristic O H stretching bands associated with a well crystallised kaolin at 3695, 3669, 3650 and 3620cm-l. As milling time increased, the intensity in the 2800 to 3400cm-' region, which can be attributed to the presence of H-bonded OH species, increased dramatically. Moreover, the four characteristic bands began to converge and alter in relative intensity after only 3 min milling. After 30 min milling the 3669 and 3650cm-1 bands were no longer visible and the intensity of the hydrogen bonded OH stretching was significantly increased. The increase in H-bonded OH was reflected in the O H bending region (Figure 2b). The peaks at 1930 and 1823cm-l are combination bands and can be used as internal markers of intensity. Thus, Figure 2b shows that the intensity in the OH bending region also increased dramatically with milling time. Hence, the DRIFTS spectra recorded at room temperature correlated well with the TG traces shown in Figure 1 in that there was much more adsorbed water present as the milling time progressed. These results are in accord with the results of others.' -6 Variable temperature DRIFTS revealed, as anticipated, that increasing the sample temperature reduced the intensity in both the OH bending and O H stretching regions, as illustrated for the 30 minute milled sample in Figures 3 and 4. This also emphasised an OH bending band at 1670cm - which remained prominent to high temperatures (Figure 3). This band position is indicative of strongly H-bonded water. This prompted an investigation into whether this band existed in the other milled samples. Consequently, DRIFTS spectra recorded at increasing temperature for each of the samples were fitted and representative fits for the OH stretching (Figure 5 a,b) and O H bending region (Figure 5 c,d) of the 10min milled sample are presented below. This showed that the 1670cm-' band (band w in Figure 5 c,d) was present in the 3 and 10 minute milled samples but was more prominent in the latter.

The Nature ($Adsorption Sites on Unrefined and Bull Milled Kaolin

84

(a>

b" .3 S 0

r b a n C

0.0

3400

3800

3000 Wavenumber / cm-'

(b)

0.05 A b S

0

r b a n C

e 0.00 2000

1800

1600 14( Wavenumber / cm-'

Figure 2 D R I F T S spectra for the unrefined and milled kaolin samples collected at 25 "C. (a)OH stretching region; (b)OH bending region

Figures 5a and 5b illustrate that removal of the broad hydrogen bonded OH stretching band (band g) revealed the presence of an underlying, very strongly hydrogen bonded species centred at 3050 cm-l. The gradual shift in position of the bands in the OH stretching region is illustrated for the 10 min milled sample in this figure along with the associated band intensity (Figures 6). Note that the areas of the bands near 3500 and 2700cm-' in the 10 minute milled sample do vary with temperature but that the band positions are relatively stable. The fitting procedure in the OH bending region revealed that the band positions were relatively stable as the temperature increased. Moreover, the band near 1730cm- did not diminish in intensity as the temperature was raised which indicates that it too is a structural band as are the 1930 and 1823cm-1 bands. The areas of the OH bending bands for the unmilled and 10min milled samples are shown in Figure 7. Gonzalez-Garcia et al.,I have published electron micrographs which illustrate

C. Breen et al.

85

.05

A b S 0

r b a n C

e

0 2000

1800

1400 Wavenumber I cnf'

Figure 3 The efect of temperature on the intensity in the OH bending region for the 30 m milled sample .25

1695

3623

A b S

0

r b a

n C

e 0 3800

3600

3400

3200 3000 2800 Wavenumber I cm-'

Figure 4 The eflect oftemperature on the intensity ofthe OH stretching regionfor the 30 m milled sample

the formation of spherical aggregates of kaolin as the time of milling is increased. These aggregates are made up of kaolin particles which have been reduced in size during the milling process. However, little is known regarding the forces that hold these aggregates together.

The Nature of Adsorption Sites on Unrejned and Ball Milled Kaolin

86 .4

(a) 25 OC

A

.05 A b

b $

S

0

0

r b

r b a

a n

n C

C

e

e

0

0

2000

1800

1600

Wavenumber / cm.'

.35

l

A b

(b) 350 "C

.03 A b

9

S

0

0

r b a

b

r d

n

n

c

c

e

e 0

0

2000

'

1600

Wavenumber / cm I

Figure 5 Representativejtted spectra for the 10 m milled sample. OH stretching region at ( a ) 25 and (b)350 "C. OH bending region at (c)25 and ( d ) 350 "C

The position and thermal stability of the 1670cm-I band together with the fact that its intensity increases with increased milling time suggests that it may be attributed to water in a very strongly hydrogen bonded environment. It is hypothesised that this band indicates the presence of a type of 'adhesive' water which holds together the aggregates of milled kaolin particles.

3.2 Kaolin Pyridine Samples The infrared spectrum of pyridine adsorbed to solid surfaces is routinely used for the evaluation of the number and type of acid site^.^-^ The rich spectrum of adsorbed pyridine can also provide information regarding the extent of hydrogen bonding that the sorbed pyridine is involved in, The presence of Bronsted acid sites is unequivocally identified by the observation of a broad band at 1540cm-' together with a strong 1490cm-1 band. In contrast the presence of Lewis acid sites are revealed by the presence of a strong, thermally stable 1450cm- band which is not to be confused with bands for physisorbed pyridine at 1435cm-' (easily removed in a flow of nitrogen or a poor vacuum) or H-bonded pyridine at 1440cm-1 (together with a band at 1590cm-I). The 1490cm-* band is much weaker for Lewis bound pyridine than for Bronsted bound pyridine.

C. Breen et al.

87

Unmilled kaolin 3600

10 minute milled 1.4 7

0.4

3600 AAAAAA

3400

-

&A& A ****OmmO

3400

*+*

A , A ~ ~ ~ ~ k A d

A A

A A

3200-

3200

3ooo

3000 -

2800

-

Q

e

\

-

a

a .".

0.7 AA:[

-3 AA.

-

....

2800 -

I

a

2600

0

I

A~A'L

...===

I

I I I H W - 1

2600

0.2

0

0

Temperature I OC

250

500

Temperature I OC

Figure 6 Comparison of the positions and areas of the H-bonded OH stretching hands for the unmilled and milled samples. The triangles, the circles and the squares correspond to handsj ; g and h in theJitted spectra in Figure 4

Unmilled .4

.3

I 0 minute milled rn

.6 .5

-

* .4

+

-2-

.3

8

I

rn

I

.2

-

I

.I

I

0

250

I

m

I . *

0

m

-1

+ -

0 ,

-

500

0

I

250

500

Temperature I "C Figure 7 The areas of the OH bending bands,for the unmilled and 10 m milled samples. The $fitled circles in the unmilled sample represent the area of the 1650cm-I band, whereas in the 10 m milled sample they represent the area of the 1670 cm-' band. The $filled squares indicate the area of the 1630cm- band in the lorn milled sample

The DRIFTS spectrum for the unmilled kaolin/pyridine sample exhibits bands associated with H-bonded pyridine (1440 and 1593cm- l ) and Bronsted bound pyridine 1540,1490 and 1609cm - (Figure 8). Raising the temperature of the sample resulted in the removal of the bands assigned to H-bonded pyridine and a gradual decrease in intensity of the bands assigned to Bronsted bound pyridine. By 250 "Cthe spectrum of sorbed pyridine had changed to that of the

The Nature of Adsorption Sites on UnreJined and Ball Milled Kaolin

88

---- kaolirdpyridine ..... kaolin

1823

.1 A b

1656

S

25 o c

1609

1441

0

r b a n C

e

c

N: I

1

2000

1 a00

I

I

1600 Wavenumber / cm-'

Figure 8 Comparison of the DRIFTS spectra for unmilled kaolin and unmilled kao1inlpyr idine

.06 A b S 0

r b a n

-

-.

C

e

f om--

_ _ _ _ _ ka o I i n/p y rid in

... ... . kaolin

0

2000

1800

1600 W avenumber / cm-'

Figure 9 Comparison of the DRIFTS spectra for 1Om milled kaolin and 1Om milled kaolinlpyridine

Lewis bound form with a strong band at 1440 cm-' together with a weak 1490cm-1 band. This implies that the Bronsted acidity is associated with the strongly bound water and as this water is removed the pyridine becomes coordinated to a Lewis bound site either nearby or at the undercoordinated A1 site produced by the removal of surface bound water. This transformation of Bronsted to Lewis acid centres is well established in catalyst chemistry as the sample

C. Breen et al.

89

temperature is r a i ~ e d . ~ The strong 1490 cm-I band together with the broad peak at 1546 cm-' clearly identifies Bronsted acid sites on the surface of the 10 minute milled sample (Figure 9). In addition the amount of water, particularly the strongly bound ('adhesive') water, which gives rise to the OH bending band at 1670cm-' was significantly reduced in intensity in the presence of pyridine and the main OH bending band was at 1630cm-I. This may indicate that the pyridine interacts with the strongly H-bonded water and removes a proton to form the pyridinium ion. As with the unmilled kaolin the intensity of the Bronsted bound pyridine bands decreased as the temperature was increased. At 250 "C there was evidence of strong Lewis bands at 1451 cm-'.

4 Conclusion Variable temperature DRIFTS analysis of the samples identified the presence of at least two distinct types of sorbed water in the milled samples. In particular, an OH stretching band at 1670cm-I increased as the milling time increased and exhibited marked thermal stability. The water giving rise to this band has been tentatively attributed to strongly bound water which is holding the ground kaolin particles in larger aggregates which have been observed here" and elsewhere.' The adsorption of pyridine identified the presence of Bronsted acid sites on the surface of kaolin at room temperature. These were converted to Lewis acid sites as the temperature was raised probably as a result of the thermal desorption of sorbed water.

References 1. F. Gonzalez-Garcia, M.T. Ruiz Abrio and M. Gonzalez Rodriguez, Clay Miner., 1991, 26, 549. 2. G. Suraj, C.S.P. Iyer, S. Rugmini and Lalithambika, A p p l . Clay Sci., 1997,12, 11 1. 3. E. Kristof, A.Z. Juhasz and I. Vassanyi, Clays Clay Miner., 1993,41, 608. 4. H. Kodama, L.S. Kotlyar and J.A. Ripmeester, Clays Clay Miner., 1989,37, 364. 5. A. La Iglesia and A.J. Aznar, J . Muter. Sci., 1996,31,4671. 6. E. Mendelovici, R. Villalba, A. Sagarzazu and 0.Carias, Clays Clay Miner., 1995,30, 307. 7. E.P. Parry, J . Catal., 1963,2, 317. 8. C. Breen, Clay Miner., 1991,26,473. 9. J.W. Ward, J . Colloid Inter$ Sci., 1968,28,269. 10. J. Ill& PhD Thesis, Sheffield Hallam University, 2000.

Computer Simulation of Water Molecules at Mineral Surfaces M.R. Warne,' N.L. Allan2 and T. Cosgrove2 'TRIPOS RECEPTOR RESEARCH, BUDE-STRATTON BUSINESS PARK, BUDE, CORNWALL, EX23 SLY, UK 2SCHOOLOF CHEMISTRY, UNIVERSITY O F BRISTOL, CANTOCK'S CLOSE, CLIFTON, BRISTOL, BSS lTS, UK

1 Introduction The nature of the interfacial structure and dynamics between inorganic solids and liquids is of particular interest because of the influence it exerts on the stabilisation properties of industrially important mineral based systems. One of the most common minerals to have been exploited by the paper and ceramics industry is the clay structure of kaolinite. The behaviour of water-kaolinite systems is important since interlayer water acts as a solvent for intercalated species. Henceforth, an understanding of the factors at the atomic level that control the orientation, translation and rotation of water molecules at the mineral surface has implications for processes such as the preparation of pigment dispersions used in paper coatings. In these proceedings we summarise the key findings from work which we have published previously.' Using 'force field' based simulation approaches we have compared the behaviour of water at both kaolinite and silica surfaces. The effects of changing the ionic strength of the water are also considered. We have examined the spatial motion distribution functions of water molecules away from the mineral surface, along with their diffusion and rotation. Comparison with experimental data is made by linking calculated diffusion coefficients to pulse field gradient nuclear magnetic resonance (PFG NMR) spectroscopy and rotational correlation times to relaxation measurements. NMR is chosen because experimentally it provides a convenient and non-invasive means for measuring molecular motion.

90

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M.R. Warne, N.L. Allan and T. Cosgrove

2 Theoretical Methodology 2.1 Static Simulations To yield the crystal structure and the lattice energy of bulk kaolinite (Figure 1), energy minimisation techniques have been employed. Under three-dimensional periodic boundary conditions, a static energy minimisation of the bulk kaolinite unit cell was performed using the Newton-Raphson method.’ The calculation was carried out at constant pressure and terminated when the change in energy had been reduced to a level of 9.65 x lop5kJ mol-l. The success of any simulation relies on the accuracy and transferability of the short range interatomic potentials. As in a recent study of m u ~ c o v i t e ,a~ set of atomic charges and potentials developed by Collins and Catlow4 have been used which also incorporate the 0-H potential added to this set by de Leeuw et al.’ in their study of hydroxylated MgO surfaces. Kaolinite is easily cleaved perpendicular to the c direction, since the interactions between the aluminosilicate layers are much weaker than the intralayer interactions. Therefore as a part of this work the structure and surface energy, y,,, of the resulting (001) surface are considered. The surface energy may be evaluated from the energy of a single layer surface block of clay in a vacuum, Us,and the energy of a portion of the bulk clay, U,, containing the same number of atoms as the surface block.

Y,,

=

us - Ub ~

A

Figure 1 Structure of kaolinite. The ‘A1 surface’ is the top layer and the ‘Si surface’ is the bottom layer

92

Computer Simulation of Water Molecules at Mineral Surfaces

For the atomistic simulation of the kaolinite interface it is assumed that surfaces are planar. Irregularities such as steps, kinks and ledges, which are present on real surfaces, are omitted for the present treatment. For kaolinite the energy of the (001) basal surface was evaluated using a suitable cell containing 425 atoms.

2.2 Molecular Dynamics Simulations Using the DLPOLY code,6 molecular dynamics simulations have been performed utilising the NVT ensemble. The surfaces of the minerals considered here have a dipole moment with acts in the direction of the z-axis (type I11 according to the Tasker classification7). To ensure the periodic simulation cell had no net dipole moment, the cells contained either two kaolinite slabs, inverted with respect to each other, consisting of 425 atoms and 820 water molecules (25.81 A x 26.70A x 77.00A), or two silica slabs in contact with 800 water molecules (28.50 A x 28.50 A x 80.00 A).The interlayer separation between two kaolinite layers is z 35-40 A.The same interatomic potentials were used as for the static simulations, allowing all atoms within the clay to move. The charges and potentials for water were those of the TIP3P/OPLS Interactions between the water molecules and the mineral surface were calculated using the Lorentz-Berthelot geometric mixing rules. For those simulations also including ions in the water layer potentials for N a + and Cl- potentials were extracted from the OPLS forcefield.**Short range terms in the interaction potential were cut off at 8.0 A,while long range coulombic interactions are calculated using the Ewald technique.12 Water molecules (density 1 g cm-3) were placed uniformly within the periodic cell. Dynamics simulations were carried out at 298 K for 100 ps. Time steps were ' ~ used with a time step of 0.5 ps. set at 0.001 ps and a Berendsen t h e r m ~ s t a t was Typically, 20 ps were used for equilibration runs, followed by production runs of 80 ps.

3 Results 3.1 Static Simulations Experimental lattice parameters for bulk kaolinite, together with those calculated in the static limit, are listed in Table l . A difference in the length of parameter b of 2.9% is the largest discrepancy, which is reasonable since our calculation relates to an idealised clay structure. Calculation of the surface energy, which is an average of the energies of the aluminium and silicon terminated surfaces of kaolinite is 48 mJ mP2.This value is in surprisingly good agreement with values predicted experimentally using adsorption isotherm film pressures14 (34-103 mJ m-2) and contact angles of solutions on the kaolinite surfaceI5 (35-96 mJ m-2).

93

M.R. Warne, N.L. Allan and T. Cosgrooe

Table 1 Calculated and experimental lattice parameters for kaolinite Lattice parameter

Simulation

Experiment

5.135 5.305 7.321 75.05 83.13 58.87

5.155 5.155 7.405 75.14 84.12 60.18

3.2 Molecular Dynamics Simulations 3.2.1 Density Profiles. A simulated density profile for water molecules (centre of mass) in one dimension along an axis perpendicular to the clay surface is shown in Figure 2(b). The scale is such that -5.5 < z < 1 and 23 < z < 29.5 corresponds to the clay slabs. Significant structural ordering of the water at both clay surfaces may be observed in Figure 2(b), as is clear by a comparison with Figure 2(a) which shows the density profile for liquid water alone in a periodic box 30 A in length (density 0.98 gcmP3).The extent of ordering of adsorbed water is in general agreement with experimental data for water at clay surfaces.' Solid-like structural ordering is observed out to approximately 10A from the surface. Figure 2(b) shows that the clay layers are not quite symmetric in the periodic cell. It is interesting to note that in the smaller gap between the clay layers, ordering of the water extends throughout the gap. The appearance of ordering appears to be associated with the relatively ordered arrangement of oxygen atoms and OH groups at the clay surfaces. Figure 3 shows a representative snapshot of the M D simulations carried out for Figure 2(b), which reflects the alignment of the water molecules at the two surfaces. At the Si surface one or both water hydrogens may be involved in H-bonding to the oxygens of the silicate layer. At the A1 surface all the atoms in the water molecules can take part in hydrogen bonding to the surface O H groups. The ordering at the kaolinite surface is striking when compared with similar calculations for the amorphous silica surface (Figure 2c) which shows the ordering is substantially reduced relative to kaolinite. It is tempting to associate this disorder with the disorder at the silica surface; this is consistent with the snapshot in Figure 4. A change in the structural ordering of the water at the kaolinite and amorphous silica surfaces is observed in Figures 5a and 5b show with increasing ionic strength. In these calculations one Na+ ion and a charge compensating C1- ion were introduced into the solvent. As might be expected the density profile at the kaolinite surface changes markedly, with a significant decrease in the peak intensity close to the surface, but there is still substantial ordering. There are far fewer changes above the amorphous silica surface. Figure 6a shows the density profile for kaolinite including a larger number of ions in the simulation box (lONaS, 10 Cl-), equivalent to an ionic strength of -0.4mol kg-l. The ordering persists even at this much higher concentration, although the number of water 611

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15

20

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30

35

M.R. Warne, N.L. Allan and T. Cosgrove

95

Figure 3 Representative snapshot of the M D simulations carried out for Figure 2(b)

molecules at the surface is significantly reduced. The reduced number of molecules at the surface may be accounted for by the displacement of water by Na+. Figure 6b shows the density profile for amorphous silica with 10N a + and 10 C1ions. Reflecting our previous observations, it can be seen that there is little change in the ordering of the water molecules at the surface. 3.2.2 Self Diflusion Coeficients. exploiting Einstein’s relation:

Diffusion coefficients may be estimated by

where the diffusion coefficient of the water molecules, D,is related to the mean square displacement. The value for TIP3P bulk water, 3.9 x m2 s-l, is in Figure 2 (opposite) (a) One-dimensional density profile of the centres of mass of water molecules in bulk water. (b) Density profile of the centres of mass of the water molecules along the z-axis (perpendicular to the surface) for kaolinite. (c)Density projile of the centres of mass of the water molecules along the z-axis (perpendicular to the surjiace)for amorphous silica

96

Computer Simulation of Water Molecules at Mineral Surfaces

Figure 4 Representative snapshot ojthe M D simulations carried out.for Figure 2(c)

agreement with that from previous work (3.9 x lOP9m2s-l), but is somewhat in excess of the experimental value of 2.4 x 10-9m2s-1.2 This faster diffusion of simulated water may be a reflection that the density of molecules (0.98g cm- 3, in the periodic cell is slightly lower than that found experimentally (1.00g ~ m - ~ ) . For both kaolinite and amorphous silica surfaces, the average diffusion coefficients for all water molecules in the simulation cell drop by over an order of magnitude, reflecting the marked ordering of the water molecules. Assuming that fast exchange of water between the bulk and surface regions occurs,18 it would be expected that diffusion of water molecules in the surface region was even slower than the average value for the simulation box. Increasing the ionic strength of the solution has a much smaller effect on the average diffusion coefficients, for both kaolinite and silica, as shown by the remaining entries in Table 2. 3.2.3 Dynamics of Reorientation. The approach of Impey, Madden and McDonald'' is used to consider the dynamics of reorientation for water molecules. The time autocorrelation function of the second Legrendre polynomial P, of the angle subtended by the intramolecular H-H bond vector at time t with respect to its position at time t = 0 is calculated: C2H-H(t)= (P2[eH-H(t)eHpH(0)])

(3)

97

M.R. Warne, N.L. Allan and T. Cosgrove

(4 1

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1

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1

1

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Silica

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.*x

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-5

0

5

10

15

20

25

30

35

Z-Axis I A

Figure 5 (a) Density profile of the centres ojmass of the water molecules along the z-axis (perpendicular to the surface)for kaolinite with one Nu+ and one Cl- added to the simulation box. (b) Density proJlle of the centres of mass ojthe water molecules along the z-axis (perpendicular to the surface).for amorphous silica with one Nu+ and one C1- added to the simulation box

e is a unit vector with the same direction as the H-H interatomic vector and ( ) represents the average over all molecules at time t. At large t (> 5 ps) least squares fitting of CZHH(t)to the exponential function Apexp( - t / t C ) allows us to determine the rotational correlation time, z, given by Aptc.A value for z, of 2.2ps is predicted for TIP3P water, slightly lower than the value of 2.7ps predicted by experiment. O

~

~

98

Computer Simulation of Water Molecules at Mineral Surfaces

-.

4.5 -4.0 --

,

Water

Water

4-b

Silica

Silica

_-

m

E 3.5

. 0

M

8

3.0

Y

2.5 ru

x 2.0 .C1 Y

8 n

1.5 1 .o

0.5

0.0 -35 -30 -25 -20 -15 -10 -5

0

5

10

15

20

25

30

35

Z-Axis I A

Figure 6 ( a ) Density projile of the centres of mass of the water molecules along the z-axis (perpendicular to the surface)jiw kaolinite with ten Na+ and ten C1k added to the simulation box. (b) Density projile of the centres of mass of the water molecules along the z-axis (perpendicular to the surface)for amorphous silica with ten Nu+ and ten C1- added to the simulation box

Rotational correlation times are listed in Table 3. As with the diffusion coefficients, the effect of the ordering imposed by the clay surface is clearly evident,

3.2.4 Average N M R Relaxation Times. The NMR relaxation time for water is predicted using the approach of Carrington and McLachlan.20Pure water can be

99

M.R. Warne, N.L. Allan and T. Cosgrove

Table 2 Average selfdiflusion coeficients (D) of water in various environments Model

Simulated Diflusion CoefJicient (m2s- l )

TIP3P Bulk Water Water at the kaolinite interface Water at the kaolinite interface + 1 (Na+Cl-) Water at the kaolinite interface + 10 (Na+Cl-) Water at the silica surface Water at the silica surface + 1 (Na+CI-) Water at the silica surface + 10 (Na+Cl-)

3.9 9.6 7.5 6.9 2.6 2.7 3.1

x 10-9 x lo-" x lo-"

x lo-" x x x lo-"

Table 3 Average rotational correlationfunctions (7,) of water in various environments ~

Model

Rotational correlationfunction (ps)

TIP3P Bulk Water Water at the kaolinite interface Water at the kaolinite interface + 1 (Na+Cl-) Water at the kaolinite interface + 10 (Na+Cl-) Water at the silica surface Water at the silica surface + 1 (Na+Cl-) Water at the silica surface + 10 (Na+Cl-)

2.2 23.3 33.5 31.9 85.1 95.5 81.0

described quantum mechanically as a particle with 1 = 1 since the total spin angular momentum is conserved during molecular tumbling. For ~ ~